Regeneration

Regeneration was the process by which Time Lords and others renewed themselves, causing a complete physical and often psychological change. It could happen because of severe illness, (TV: Planet of the Spiders, The Caves of Androzani, The End of Time, Day of the Moon; AUDIO: The Brink of Death) old age/fatigue (TV: The Tenth Planet, The Day of the Doctor, The Time of the Doctor) or injury. (TV: Logopolis, Time and the Rani, Doctor Who, Utopia, Let's Kill Hitler, Hell Bent, The Doctor Falls, Twice Upon a Time, PROSE: Interference - Book Two) It could also be invoked by choice, whether voluntary (TV: Destiny of the Daleks, The Twin Dilemma, The Night of the Doctor, Nightmare in Silver) or involuntary. (TV: The War Games)

Conversely, regeneration could be prevented by choice, although choosing not to regenerate from fatal damage was essentially an act of suicide. (TV: Last of the Time Lords) Although, a Time Lord could delay the actual change in appearance and enter "a state of grace" for a brief period of time. (TV: The End of Time, The Doctor Falls, Twice Upon a Time) Precise damage to a Time Lord could also cause a delay in the regeneration starting. (TV: Doctor Who, The Doctor Falls)

How regeneration worked
Different explanations were given for the process of regeneration.

One theory held that Cardinal Rassilon had been investigating a method of regenerating decayed and diseased tissue via a series of self-replicating, biogenic molecules. The cells of a Gallifreyan body would be repaired, restored and re-organised, resulting in a wholly new physical form. The brain cells would also be rearranged, though to a lesser extent; the new incarnation would retain the memories of the former incarnation, though the personality of the Time Lord or Lady could change, the degree of this change depending upon the Time Lord or Lady in question. Rassilon intended this mechanism only for the Gallifreyan elite. He also input a parameter of twelve regenerative cycles to avoid decaying biogenic molecules and also to limit total immortality, as he deemed this to be dangerous. (AUDIO: Zagreus)

Another explanation stated that Time Lords had triple-helix DNA: the third strand was added by Rassilon to enable regeneration. (PROSE: The Crystal Bucephalus)

A third account suggested that the Looms gave Gallifreyans the ability to regenerate, and that the earlier womb-born generations could not. (PROSE: Lungbarrow)

Upon observing the similarities between Time Lord regeneration and the rapid healing of the Mal'akh, Richard Francis Burton theorised that early Time Lords obtained regenerative abilities by knowingly infecting themselves with the Yssgaroth taint. (PROSE: The Book of the War)

Exposure over billions of years to the Untempered Schism contributed to the Time Lords' ability to regenerate. (TV: A Good Man Goes to War)

One Gallifreyan wrote that regeneration was a natural process for all Gallifreyans, and was not exclusive to the Time Lords. (PROSE: A Brief History of Time Lords)

Details
Time Lords released massive amounts of a hormone called lindos in moments of extreme trauma, and it was this hormone which triggered regeneration. Newly regenerated Time Lords could be identified by elevated levels of lindos in their system. (PROSE: The Twin Dilemma, AUDIO: Unregenerate!)

Time Lords were also said to have "packets" of regeneration energy in their bodies, one for each life. These packets could be physically removed from a Time Lord's body, essentially robbing them of their regenerations. (TV: Mawdryn Undead)

During regeneration, a Time Lord experiences a surge of pure regnerative energy, as their entire system was rewritten and their universe moulded into a new shape, with their very biodata being rewritten in the fabric of space-time. (PROSE: Unnatural History) Unfortunately, their immune systems were seriously weakened as the process took place, with the result that they could be infected by viruses such as the Faction Paradox biodata virus that would normally have had no effect on them. (PROSE: Interference - Book Two) On a wider scale, regenerations were so important to a Time Lord's timeline that even minor temporal paradoxes during the process of regeneration could have devastating consequences. (AUDIO: Prisoners of Fate, TV: Twice Upon a Time)

The Fourth Doctor explained that he still had the same body as his first incarnation; it had just been restructured a lot since then. He agreed that the process of regeneration went against Newton's Third Law, and added that he did violate a few laws "now and then". (PROSE: Legacy of the Daleks)

Appearance
During a regeneration, a Time Lord's body could shine with milky white light, (TV: The Tenth Planet, PROSE: The Indestructable Man) a swirl of rainbow colours, (TV: The Caves of Androzani, Time and the Rani, Utopia) crackle with electricity, (TV: Doctor Who) or with a discharge of golden energy, which could vary in speed and intensity from being brief and contained to explosive and capable, in extreme circumstances, of causing damage to the nearby area. (COMIC: The Forgotten, TV: The Parting of the Ways, Utopia, The Stolen Earth, The End of Time, Day of the Moon, Let's Kill Hitler, The Night of the Doctor, The Day of the Doctor, The Time of the Doctor, Hell Bent, The Lie of the Land, World Enough and Time, The Doctor Falls, Twice Upon a Time) In other cases, there was no apparent energy discharge at all, just a fade away to the next incarnation. (TV: Planet of the Spiders, PROSE: The Ancestor Cell, The Touch of the Nurazh) In the case of the Doctor's fourth regeneration, the Doctor appeared to merge with and become the Watcher, after which he transformed from the Watcher into his fifth incarnation. (TV: Logopolis) On the occasion that the Doctor had just been granted a brand new regeneration cycle, he initially emitted a large eruption of energy in an almost liquid form. Sometime later, when his actual shift to a new body occurred, there was only a very short, almost negligible discharge of energy. (TV: The Time of the Doctor)

Some regenerations occurred with other individuals in close proximity. These times, the energy from the Doctor's body wasn't particularly violent. (TV: The Tenth Planet, Planet of the Spiders, Logopolis, The Caves of Androzani, Time and the Rani, The Time of the Doctor) However, from their eighth incarnation onward, the Doctor warned anyone in close proximity to a regeneration to keep away. This happened with members of the Sisterhood of Karn prior to the Eighth Doctor transforming into his so-called War Doctor incarnation, (TV: The Night of the Doctor) when the Ninth Doctor received a brief surge of regenerative energy in the presence of Rose Tyler, (TV: The Parting of the Ways) and when the Eleventh Doctor realised that Mels was regenerating. (TV: Let's Kill Hitler) When the Tenth Doctor regenerated during the 2009 Dalek invasion of Earth, Captain Jack Harkness similarly warned Rose Tyler to stay away from the Doctor when his regeneration started. (TV: The Stolen Earth) Why the Doctor thought this was necessary became clear when the Tenth Doctor's regeneration into the Eleventh caused enough damage to the TARDIS to force a complete reconstruction into a different design. (TV: The End of Time) When he began his thirteenth regeneration, it was the most destructive of all — emitting a shock-wave resembling a thermonuclear blast. The town of Christmas was leveled, Dalek fighter pods were blown out of the sky, and a Dalek Saucer was destroyed. Because the Doctor didn't have time to warn the citizens of Christmas of the danger from regenerating, his companion Clara Oswald ushered the people into the bell tower to take shelter. The Daleks that remained on the street were blown to pieces. (TV: The Time of the Doctor) The Twelfth Doctor was able to tell that his fourteenth regeneration would be explosive, (PROSE: Twice Upon a Time) and it eventually caused enough damage to the TARDIS for it to malfunction. The control room exploded, sending the Thirteenth Doctor plummeting from its doors. (TV: Twice Upon a Time)

Physical and mental change
During regeneration, there were the genetic equivalent of "bit errors" in the DNA of the regenerating cells. The Time Lord would change in appearance, height, mass or apparent age. The personality would also change; even the cells and chemistry of the brain regenerated, although their blood type would remain the same in all their lives. (PROSE: The Eight Doctors) Allergies and dominate limbs could also change between regenerations. (AUDIO: The Lady in the Lake) According to the Sixth Doctor, a Time Lord's basic personality traits remained unchanged throughout all their lives, but the balance of said traits could be affected by regeneration. (AUDIO: The Sirens of Time) With each incarnation, the Doctor's memory worked differently, (PROSE: Nothing O'Clock) with some of their memories from before regeneration being lost even after the new incarnation had mentally stabilised, unless they were specifically reminded of relevant events. (TV: Castrovalva, Doctor Who, Deep Breath) When he fell unconscious after being exposed to radiation, the Sixth Doctor 'heard' the voice of his successor before the regeneration began in full, leaving him assured that he would regenerate despite the circumstances of his death, the voices of the two Doctors briefly 'merging' in his mind as they each noted that their life was far from over. (AUDIO: The Brink of Death) When the Eighth Doctor was stabbed by Charley while he was possessed by anti-time, he nearly 'died' for real due to his current grief, but found himself conversing with the personas of his immediate three predecessors who helped him realise what had happened and the need to return to life to stop Rassilon controlling the power of anti-time. (AUDIO: Zagreus) When the amnesiac Eighth Doctor was in a coma after his chest was crushed by sandbags, during his coma fantasy he experienced a feeling that he compared to the idea of his body as a coat that he could take off and don another body around a corner, but in this instance the Doctor did not need to regenerate as his current link to his 'adversary' Sabbath would sustain his life even after such serious injuries. (PROSE: Camera Obscura)

One source stated that Time Lords were born with just one heart and grew a second heart on regeneration. This included the Doctor, who in their first incarnation had only one heart. (PROSE: The Man in the Velvet Mask) Other accounts showed Time Lords having two hearts in their original incarnation, such as the Doctor (AUDIO: Frostfire, The Abandoned) and Jenny. (TV: The Doctor's Daughter) Regeneration also changed the location of said hearts. (TV: Dalek, The Power of Three) According to the Eleventh Doctor, every regeneration was painful, (TV: Death of the Doctor) with the Seventh once describing regeneration as a good and bad feeling in the same way driving a car very fast was a good and bad feeling, enjoying the exhilaration of the process but knowing you were going to "die" at the end. (PROSE: The Room With No Doors) Klyst assured Louis following his first regeneration that the first was always more painful than subsequent ones. (AUDIO: Unregenerate!)

The Doctor, the Master, Rindle, and Lake each had female incarnations, while the majority of their incarnations were male. (TV: Twice Upon a Time, Dark Water, AUDIO: The Lady in the Lake) The General was usually a woman, but one of their incarnations was a bald white male. (TV: Hell Bent) Skin colour could also change between regenerations. (TV: Let's Kill Hitler, Hell Bent, PROSE: Engines of War, AUDIO: The Next Life, The Lady in the Lake)

A change in gender, size and skin colour appeared to be fairly common, the Twelfth Doctor stating that he was "one of those stuck-in-a-rut Time Lords who always [had] the same model of body." (PROSE: Twice Upon a Time)

More extreme changes were also possible. Cavisadoratrelundar regenerated a complete body after being decapitated—although the process was cut short when she was stabbed through both hearts (the one that she already possessed and the one that she was growing as she regenerated)—and her lover Gandarotethetledrax became a kind of human-Silurian hybrid in appearance, when he regenerated in the realm of Avalon mere hours after his previous change. (PROSE: The Shadows of Avalon) The Doctor's eleventh incarnation stated clearly that he could become "anything"; (TV: Death of the Doctor) the Ninth Doctor suggested two heads or none were both possible. (TV: The Parting of the Ways) According to the Valeyard, there was also a risk of emerging from a regeneration as a Time Tot rather than an adult Time Lord. (AUDIO: Trial of the Valeyard) River Song's second incarnation started off as a baby, (TV: Day of the Moon, Let's Kill Hitler) while Rassilon actually had the opposite happen to him, changing from the body of a middle-aged man to that of an elderly man. (TV: Hell Bent) The Master's sixteenth incarnation resembled a small male child at the start of the Time War. (COMIC: The Then and the Now)

When the Eleventh Doctor lost his leg during the Siege of Trenzalore, he grew a new one after his body was 'reset' as his new regeneration cycle began. The Twelfth Doctor had both legs once he had regenerated. (TV: The Time of the Doctor, Deep Breath)

In nearly all cases, Time Lords remained humanoid after regeneration; one Time Lord, Lord Cardinal Zero, regenerated into an avian lifeform as a result of the poison which triggered the regenerative process. (AUDIO: Spring) Though a healthy body seemed to be the default, the Doctor's eleventh incarnation made it a priority — even amidst serious damage to his TARDIS — to immediately conduct a physical inventory to make sure he still had two legs and sufficient fingers, eyes, ears, a nose, chin and hair. (TV: The End of Time)

In the early days of regeneration, it was possible for fragments of other DNA to be incorporated into the new incarnation if, for example, a Time Lord had recently eaten or spent a great deal of time around other species; the early Gallifreyan priest I.M. Foreman suffered from this problem throughout his regenerations, each incarnation becoming more and more inhuman as more foreign DNA was incorporated into the process. (PROSE: Interference - Book One, Interference - Book Two)

As well as the obvious benefits of purging Time Lords of any poisons or diseases that might have caused their deaths, regeneration could have more subtle benefits. As each Time Lord accumulated artron energy throughout their lives, regeneration reset the artron energy levels in their bodies to a pre-set level to prevent it rising to a point where the radiation could threaten them. (PROSE: Empire of Death) When the Sirens of Time attempted to make the Doctor their agent, while responding to the Sirens' call more than once would have brought the Doctor permanently under their control, the Doctor was still a free agent as he had responded to their call three times in three different incarnations, suggesting that regeneration would purge the Sirens' influence. (AUDIO: The Sirens of Time) However, this purge would not include 'assets', as the Seventh Doctor retained an immunity to the Swarm virus despite only receiving that immunity in his fourth incarnation. (AUDIO: Revenge of the Swarm) Some negative influences could not be purged by regeneration; while the Fourth Doctor was able to resist succumbing to the Breath of forever (AUDIO: Destroy the Infinite), he was still susceptible to the influence of the Eminence in his sixth incarnation. (AUDIO: The Seeds of War)

After-effects
Regeneration mixes up the synapses in the brain, sometimes resulting in negative after effects. (AUDIO: The Lady in the Lake) During the first few hours of the regeneration, the Time Lord might suffer from confusion, erratic behaviour, extended periods of unconsciousness, or memory loss. (TV: Spearhead from Space, Robot, Castrovalva, Doctor Who, The Christmas Invasion, AUDIO: The Lady in the Lake, et al) Motor control could be impaired, the Eleventh Doctor expressed difficulty "steering" his new body, having walked into a tree minutes earlier, and a Time Lord could suffer random spasms as the regeneration settled. (TV: The Eleventh Hour)

The Doctor, in particular, seemed to be highly susceptible to post-regenerative side-effects. After their first regeneration, the Second Doctor had some confusion over his own identity, although he adjusted to his new body reasonably quickly. (TV: The Power of the Daleks) Their third incarnation was incapacitated for some time after a forced regeneration by the Time Lords. (TV: Spearhead from Space) Likewise, their fourth incarnation suffered acute delirium and memory loss, and was placed under bed rest for a few days until he regained consciousness. (TV: Robot) It took a while for the newly regenerated Fifth Doctor to remember his own identity, experiencing fainting spells and sudden spurts of alternating energy and fatigue until he stabilised. (TV: Castrovalva) Their sixth incarnation confused myths with reality, resulting in him trying to strangle Peri before coming to his senses, and experienced erratic mood swings of enthusiasm or depression until he became reasonably stable. (TV: The Twin Dilemma) The Seventh Doctor was sufficiently weakened that the Rani was able to effectively brainwash him. (TV: Time and the Rani) The Eighth Doctor suffered from amnesia and emotional instability, after having been declared dead for some time prior to regenerating due to the anaesthetic that had been used on his predecessor. (TV: Doctor Who) The War Doctor, having been born from an elixir that assisted the process, showed no immediate signs of post-regenerative distress, despite his predecessor being technically dead at the time of regeneration, but did look at his new face in a reflection. (TV: The Night of the Doctor)

The Ninth Doctor mentioned being "not quite calibrated" after regeneration, and likened himself to a soft shell crab waiting to harden, (PROSE: The Beast of Babylon) and the newly regenerated Tenth Doctor spent hours unconscious after an initial burst of wild enthusiasm. This incarnation also openly confessed to having no idea what kind of person he had initially turned into, stating he could be anything including "a gambler, a fighter, a coward, a traitor, a liar, a nervous wreck." (TV: The Christmas Invasion) Immediately post-regeneration, the Eleventh Doctor craved various foods, which he hated upon tasting them. (TV: The Eleventh Hour) At the start of a new regeneration cycle, the Twelfth Doctor experienced severe memory loss, forgetting how to pilot the TARDIS, (TV: The Time of the Doctor) and spent the next few hours in a delirious state. He was unable to remember names and distinguish people from each other to the point that he confused Clara Oswald with both Handles and Strax. (TV: Deep Breath)

On two occasions the Doctor crashed the TARDIS following regeneration, first when the Tenth Doctor compulsively sped up the TARDIS and hit it against a wall in the Powell Estate, (TV: Children in Need Special, The Christmas Invasion) then when the Tenth Doctor's regeneration into the Eleventh damaged the TARDIS, causing it to crash land into a garden shed in Leadworth. (TV: The End of Time, The Eleventh Hour) The Thirteenth Doctor was thrown out of her TARDIS after damage dealt by her regeneration caused an explosive malfunction. (TV: Twice Upon a Time)

As regeneration involved completely rewriting a Time Lord's biodata and exposing it to the Time Vortex, for a time, the Eighth Doctor speculated that his raw biodata was responsible for the transformation of his companion Samantha Jones from her original dark-haired, self-destructive self into his blonde-haired 'perfect' companion, as his biodata was particularly exposed after his traumatic regeneration and the opening of the Eye of Harmony. However, Professor Daniel Joyce dismissed this theory for blonde Sam's existence as impossible as the Doctor could never have had that degree of unintentional influence on a complete stranger, although he admitted that the basic concept of Time Lord biodata rewriting another person's biodata wasn't completely impossible if there was greater pre-existing contact between them. (PROSE: Unnatural History)

A Zero Room could help with the regeneration-recovery process, as it removed all outside distractions. (TV: Castrovalva) After his first regeneration, the Second Doctor implied that the TARDIS itself helped the process along. (TV: The Power of the Daleks) After regenerating, the Tenth Doctor said he needed the TARDIS. In some cases proper rest helped a Time Lord's mind heal, but waking too soon prolonged the need of it; it was referred to as a "neural implosion". He later explained that some tannin in tea helped heal the synapses, accelerating the healing process. (TV: The Christmas Invasion)

Were a Time Lord knocked unconscious, the whole process might start all over again. (PROSE: The Power of the Daleks) Indeed, the Fourth Doctor thought that he had regenerated again without noticing when the Brigadier told him he had changed (referring to his clothes, not his appearance). (TV: Robot) However this was not a certainty, (TV: The Eleventh Hour) as it seemed that sleep would help a Time Lord recover from post-regenerative delirium. (TV: Deep Breath) After a while, the Time Lord's body would settle down, though they could regrow lost limbs within the first fifteen hours of the regeneration due to residual energy. (TV: The Christmas Invasion, Let's Kill Hitler) The residual energy also provided a Time Lord with greater endurance; the Twelfth Doctor fell out of a tree, but remained unaffected by it. (TV: Deep Breath)

Even after the physical transformation, changes might occur. The Doctor was excessively tired after their third regeneration, falling asleep in many odd locations. (TV: Robot) The Fifth Doctor's hair went from longer to shorter to longer in the space of a few days. (PROSE: Cold Fusion) For a short time after regenerating, a Time Lord displayed greater strength than usual; the Doctor's fourth incarnation was able to karate-chop a brick in half shortly after his regeneration, though he failed to repeat the action once fully recovered (TV: Robot) while their eighth incarnation broke down a steel door with his bare foot immediately following his regeneration. (TV: Doctor Who) Melody Pond, following her second and final regeneration, not only used her regenerative energy to survive a hail of gunfire by Nazi soldiers, but channelled it into a focused blast of energy that knocked out the entire squad. (TV: Let's Kill Hitler)

Forced regenerations could also result in Time Lords losing some of their skills or memories in various incarnations. Some of these changes could be minor, such as the Fourth Doctor reflecting that he was no longer able to make successful soufflés, (PROSE: Heart of TARDIS) while on two occasions, later Doctors were forced to mentally regress back to their third persona, when they found themselves facing a situation where the Third Doctor's skills would be better-suited to handle the crisis than their own abilities were. This happened during their sixth and seventh incarnations. The Sixth Doctor required the Third's skills at hand-to-hand combat (PROSE: Stage of Change) while the Seventh felt that his technical expertise would be useful. (PROSE: Timewyrm: Genesys) The Fifth Doctor also notably lacked the hand-to-hand combat skills of his two immediate predecessors, though he remained a fairly capable swordsman, (TV: The King's Demons) and the Twelfth Doctor expressed uncertainty about his ability to use a motorbike early in his life as he hadn't had a chance to test his current capabilities. (PROSE: The Crawling Terror) Post-regenerative amnesia could also be a problem, preventing the Doctors' recall of crucial information, such as the Sixth Doctor forgetting about his predecessor's encounter with Katherine Chambers when he met a younger version of her despite his past self's attempt to leave a reminder, resulting in him being unable to avert Katherine's interest in acquiring Cyber-technology. (AUDIO: The Reaping, The Gathering) Personal tastes could also vary between different incarnations, such as the Fifth Doctor not being a particularly strong drinker, (AUDIO: The Kingmaker) while the Fourth (TV: The Twin Dilemma) and Sixth (PROSE: The Quantum Archangel) enjoyed the chance to get drunk, the Fourth Doctor expressing surprise at the attire he would wear in his sixth (AUDIO: The Light at the End) and eleventh bodies (PROSE: The Roots of Evil) when he caught glimpses of his future, or the Eleventh Doctor enjoying football, (TV: The Lodger) while the Twelfth considered it a boring sport. (TV: For Tonight We Might Die) The mental transformation caused by regeneration could also be a problem; when Andred decided to pose as Torvald after the two fought in the Capitol basement, Torvald dying while Andred regenerated, immediate post-regenerative trauma was so difficult that Andred didn't think about what his actions would do to his wife Leela before he was too far into his deception (AUDIO: A Blind Eye), others noting later that Andred had spent so long pretending to be Torvald that his own new personality had never asserted itself, resulting in him still expressing some of Torvald's personality traits even after his deception was exposed. (AUDIO: Lies)

However, some Time Lords were able to regenerate with little or no overt complications, as in the case of Romana (TV: Destiny of the Daleks) and the human River Song, (TV: Let's Kill Hitler) who developed Time Lord characteristics after being conceived in a TARDIS while it was in flight, (TV: A Good Man Goes to War) neither of whom appeared to experience any sign of incapacitation in their exploits immediately following regeneration. Both treated the change rather casually, and acted as normal. (TV: Destiny of the Daleks, Let's Kill Hitler) The General showed no after-effects after regenerating into her twelfth incarnation, in fact stating that she was "back to normal". (TV: Hell Bent)

Limitations
Though Time Lords could regenerate after severe injuries, regeneration was by no means guaranteed. River Song stated that a Time Lord could die permanently if killed at the right time during the regenerative process. (TV: The Impossible Astronaut) Artron energy was crucial to the regenerative process, meaning Time Lords were unable to regenerate if they did not have any artron energy in their body. (PROSE: The Banquo Legacy, AUDIO: Repeat Offender) The regenerative process could also be destroyed by acid. (PROSE: Night of the Humans) Some injuries were so serious that they could not be treated by regeneration; when the Time Lady Solenti was blinded, she spent ninety-one years uncertain if she would ever regain her sight, although a telepathic exchange with a pan-dimensional race allowed her to confirm that the neural pathways for sight in her brain were still intact and she would be able to see again once she regenerated, even if she decided that she would not regenerate just to regain her vision. (PROSE: The Suns of Caresh) Certain energy weapons could also destroy the regenerative process. In an alternate reality, a Raxacoricofallapatorian used an energy weapon that destroyed the Tenth Doctor's regenerative process, preventing his regeneration into the Eleventh Doctor. (COMIC: Four Doctors) Martha Jones also had a gun that, when inserted with four distinct chemicals, could apparently prevent regeneration, as she reportedly planned to do to (the existence of this gun was actually a ruse created by the Tenth Doctor to distract the Master from their real plan, although the fact that the Master believed it suggests that such a weapon was theoretically possible). (TV: Last of the Time Lords)

While a Time Lord could delay the regenerative process, it was not entirely preventable. The First Doctor tried to halt the process, although he eventually conceded to it, having lost consciousness because of the resulting energy loss it caused. (TV: Twice Upon a Time) The Tenth Doctor, by comparison, exhibited greater control, having managed to hold back his regeneration in order to visit all of his previous companions, (TV: Death of the Doctor) but it caused him great pain, resulting in him collapsing to the ground and eventually submitting to the process, the resulting regeneration destroying the main control room. (TV: The End of Time) The Twelfth Doctor tried to do likewise, holding it back for several hours. However, he suffered from a loss of energy, constantly stumbling, and eventually lost the strength to stand up in the TARDIS. Like the Tenth's, his regeneration also resulted in the TARDIS taking extreme damage. (TV: Twice Upon a Time)

The Tenth Doctor tried to attach his brain to the CAL Data Core, believing he could regenerate and provide the extra memory space although River pointed out it would outright kill him by burning out both of his hearts, so she took his place. (TV: Forest of the Dead) The Master's upgraded laser screwdriver could disable the regenerative process if its target received a full blast on its maximum setting. (TV: The Doctor Falls) While not explicitly demonstrated, the Master at least believed that Janis thorns could prevent regeneration, as he went to great lengths to take control of Leela so that she would use the thorns against the Fourth Doctor, the Doctor only surviving this experience as he was able to help Leela break the Master's programming in time for her to inject him with the antidote. (AUDIO: The Evil One) River Song's lipstick could also prevent regeneration (although it was later revealed that this detail was unnecessary as the Doctor had already used up all twelve of his lives) (TV: The Time of the Doctor), causing a long and painful death that would take roughly 36 minutes, the Doctor only being cured when River sacrificed her own remaining ten regenerations to save him. (TV: Let's Kill Hitler) The regenerative process also needed time to start meaning that certain deaths could happen too quickly for the body to regenerate, as evidenced by how an alternate version of the Tenth Doctor died because he was drowned confronting the Empress of the Racnoss. (TV: Turn Left) When facing the Word Lord, the Seventh Doctor sacrificed himself to stop Nobody No-one by using a tablet linked to the Trans-Galactic net to hit Nobody with the full force of the stories collected about himself, at the cost of frying his own mind, causing so much damage that he expressed a belief that this would kill him even before the Word Lord tricked a UNIT soldier into declaring 'Nobody can stop the Doctor from regenerating' which gave the Word Lord the power to do just that. (AUDIO: A Death in the Family) Fortunately, this sacrifice was undone when the Seventh Doctor's future self- currently present at the death of his past self after his own attempt to trap the Word Lord- set up a complex plan that allowed Ace to subvert the Word Lord's power to rewrite reality so that she could undo the Doctor's death, restoring him to life in his seventh body with no need for regeneration.

According to some accounts, a blast from staser fire could prevent regeneration. Maxil implied this, (TV: Arc of Infinity) and Gandar later explicitly noted that a staser pistol was one of the few weapons capable of killing a Time Lord immediately. (PROSE: The Shadows of Avalon) However, the Twelfth Doctor shot the General once with the sidearm of the President's personal security, triggering the General's regeneration. (TV: Hell Bent) Several Time Lords died without regenerating when they were killed by a TCE. (TV: The Deadly Assassin) An alternate version of the Sixth Doctor was killed permanently by a Dalek, but this Doctor had been locked away in the Tower of London for decades after his legs had been amputated, and his mental state had significantly deteriorated, to the point that he may have wanted to die to escape his nightmarish existence. (AUDIO: Jubilee)

Stabbing or shooting a Time Lord through both hearts at the same time would kill them. (PROSE: The Shadows of Avalon, World Game) recommended eight snipers, with two trained on each of her hearts and three for her brain stem, so that Clara Oswald could feel safe while speaking to her, noting that they'd have to "switch [her] off fast" before she could regenerate. (TV: The Magician's Apprentice) Plans to execute a Time Lord relied on technology that would destroy both hearts, sever all three brain stems, and deliver a cellular shock to prevent regeneration, after which the subject would be left in a quantum vault for no less than a thousand years to ensure that regeneration would not occur. (TV: Extremis) Arkhew was unable to regenerate after being strangled by Owis. (PROSE: Lungbarrow)

The Eighth Doctor was killed when the ship he was trying to escape from crash landed on Karn. He was later restored temporarily to life by the Sisterhood of Karn and provided with an elixir to trigger the regenerative process. (TV: The Night of the Doctor) The Ninth Doctor showed the same level of concern as his human companions did towards falling victim to the Gelth and the Empty Child's plague (though the latter caused a hive mind mutation rather than death), suggesting regenerating was not an expected outcome. (TV: The Unquiet Dead, The Empty Child/The Doctor Dances) When surrounded by Daleks, the Doctor was under the belief that he would die for good, remarking that "maybe it's time". (TV: The Parting of the Ways) Copies of the Twelfth Doctor died from injuries too strong to regenerate received from contact with the Veil inside the Doctor's confession dial. However, the Doctor claimed that even if a Time Lord failed to regenerate, every cell in their body will individually keep trying to regenerate, and "dying properly can take days", giving each copy time to use the last of their energy to trigger a teleportation device and "create" a new body for their essence to continue trying to escape. (TV: Heaven Sent)

Certain environments could also be dangerous for regeneration, with the Fifth and Eighth Doctors going to great lengths to avoid dying in space, (PROSE: Imperial Moon, The Taking of Planet 5) the Eighth Doctor recalling horror stories of Time Lords regenerating in such an environment, burning themselves out as their new bodies became increasingly twisted trying to "evolve" into something that could cope in a vacuum. However, safe regeneration from exposure to a vacuum was possible if the subject was returned to an oxygenated environment before the process initiated, with one Time Lord regenerating normally after being cast adrift in space before the War Doctor materialised his TARDIS around him and left him to regenerate in the ship's Zero Room. (PROSE: Engines of War) The Fifth Doctor once noted that the important thing in regeneration was that the brain remain intact, with an early Gallifreyan being killed for good when she was shot in the brain at close range with a 26th century weapon; (PROSE: Cold Fusion) the Eleventh Doctor needed to be healed by an external force after he was shot in the head with an 18th century revolver, but he had already exhausted all twelve of his regenerations at this point. (PROSE: Dead of Winter, TV: The Time of the Doctor)

Retro-genitor radiation was specifically created by the Daleks to inhibit regeneration. (AUDIO: X and the Daleks) Certain types of radiation could be more dangerous to Time Lords than others; roentgen radiation was relatively harmless so long as it was expelled quickly enough (TV: Smith and Jones), and the Doctor was able to hold back his regeneration for several hours after being exposed to the radiation from the Immortality Gate (TV: The End of Time), but the radiation belt around Lakertya was uniquely deadly to Time Lords while safe for human exposure (AUDIO: The Brink of Death), and the radiation around the blue crystals of Metebelis III caused so much damage to the Third Doctor that he required external aid to trigger his regeneration after spending ten years drifting in the TARDIS dying of exposure. (TV: Planet of the Spiders, Love and War)

Also, the application of various medicines, such as general anaesthetic from Earth, were known to disrupt the regenerative process. (TV: Doctor Who) The Fifth Doctor was not sure if he would be able to regenerate after contracting Spectrox toxaemia, and indeed the transition into his next incarnation occurred "Not a moment to soon". (TV: The Caves of Androzani) It was implied that particularly serious blood loss could prevent regeneration altogether; Ruath was able to regenerate after draining her blood and sending it to Yarven via her TARDIS, (PROSE: Goth Opera) but the Eighth Doctor and Romana II were both concerned that the Fourth Doctor would die for good after several vampires fed on him at once before the Eighth Doctor was able to give his past self a transfusion, also confirming that blood type remains the same across all of a Time Lord's incarnations. (PROSE: The Eight Doctors)

Due to the Time Lords' reliance on artron energy to power the regenerative process, an artron inhibitor could prevent Time Lords from regenerating, as well as limit their ability to heal from damage; when Time Lord agent Cuthbert Simpson attempted to track and trap Compassion with an inhibitor, his injuries were so serious that it took him decades to recover even after the field preventing him from regenerating was deactivated. (PROSE: The Banquo Legacy) When the Doctor's TARDIS was damaged by a vitreous time storm, the subsequent effects on the Doctor due to his link to the ship nearly killed both of them until a past Doctor was able to donate the energy needed for the ship to heal itself. (AUDIO: Mary's Story) By the Last Great Time War, the Varga plants evolved so that they possessed regeneration inhibitors which could kill a Time Lord instantly. (AUDIO: Legion of the Lost) Dalek weapons also incorporated regeneration inhibitors, with the result that Daleks could kill Time Lords instantly, even if the High Council maintained the propaganda that regeneration would still be an option if Time Lords fell in battle. (AUDIO: The Conscript)

In the early days of regeneration, the process was so disruptive to a Time Lord's DNA that regenerations were known to incorporate aspects of what the Time Lords had recently eaten before regenerating. While this defect had been corrected by the Doctor's era, the old Gallifreyan priest I.M. Foreman still had this defect in his system, resulting in his thirteen incarnations representing a wide range of genetic anomalies as he pushed his biodata envelope as far as it could go, culminating in his final incarnation "evolving" to become the vibrant new biosphere for an entire planet. (PROSE: Interference - Book Two) The Doctor once claimed to be half-human, due to his mother being one. (TV: Doctor Who) Me reinforced the idea, by explaining she thought the Doctor was The Hybrid. (TV: Hell Bent) The Doctor regenerated in the presence of humans at least seven times, making it possible that he's becoming more human. (TV: The Tenth Planet, Planet of the Spiders, Logopolis, The Caves of Androzani, Time and the Rani. The Parting of the Ways, The Time of the Doctor)

Ideally, regeneration would be undergone within a low-grade telepathic field. The presence of another Time Lord was recommended to assist with any difficulties, and the newly regenerated Time Lord best remained in a state of total tranquillity for a time afterwards to allow the mind and body to properly readjust. (PROSE: Cold Fusion)

Regenerative cycle
Time Lords had a limited cycle of twelve regenerations, consisting of thirteen incarnations, after which they would suffer permanent death. (TV: The Deadly Assassin, Doctor Who, The Time of the Doctor) Time Lords could will themselves to die by attempting to regenerate when they had no more regenerations left to use, as Azmael who was hosting an evil consciousness and chose to do so to prevent it from escaping. (TV: The Twin Dilemma) The Eleventh Doctor likewise threatened to use regeneration to hold off Mr Clever from taking over his mind. (TV: Nightmare in Silver)

After their last body, a Time Lord who had exhausted their regenerations had their flesh breaking down into degenerate matter and then into random molecules, (COMIC: The World Shapers) causing their body to seemingly disappear into thin air after some time. (TV: Shada)

As with most such "rules", there were occasional exceptions to the twelve regeneration limit. The High Council offered a new regeneration cycle if he rescued the various incarnations of the Doctor from the Death Zone. (TV: The Five Doctors) Although he did not receive this award on that occasion, later in his life, he was resurrected by the Time Lords (AUDIO: Eyes of the Master, TV: Utopia) and was able to regenerate on at least three more occasions. (COMIC: Fast Asleep, TV: Utopia, Dark Water) It was indicated that the Master possessed at least one more regeneration after this as he told his future self "don't even think about regenerating" after mortally wounding her with his laser screwdriver. (TV: The Doctor Falls)

The Eleventh Doctor once flippantly claimed he could regenerate 507 times. (TV: Death of the Doctor) It was later revealed to not be true and had in fact already exhausted all twelve of his regenerations. However, the Time Lords granted him a new cycle of regenerations before the Eleventh Doctor could die for the final time, (TV: The Time of the Doctor) with the Twelfth Doctor, (TV: Kill the Moon) Rassilon, (TV: Hell Bent) and the Master (TV: The Doctor Falls) expressing uncertainty as to how many regenerations the Doctor now possessed.

Transference of regenerative energy
In an attempt to end their perpetual cycle of rebirth, Mawdryn and his crew attempted to force the Fifth Doctor to sacrifice all eight of his remaining regenerations so that they could take his 'deaths' for themselves. Although the Doctor rejected that request because he was unwilling to essentially kill himself eight times over in order to save this crew from their own mistake, he was willing to do so in order to cure his companions of an infection they had contracted from exposure to Mawdryn's crew. However, the same necessary energy was released when the Brigadiers of 1977 and 1983 made contact with each other, the temporal energies released in the subsequent paradox serving the same purpose as the Doctor's eight regenerations. (TV: Mawdryn Undead)

While regeneration was a natural Time Lord function, it was not an intrinsic part of their genetic makeup that could be acquired through simple DNA samples. When The Forge created a clone of the Sixth Doctor, the most successful clone retained the Doctor's knowledge but was incapable of regenerating, although this may have been due to flaws in the Forge's cloning technology. (AUDIO: Project Lazarus)

After was executed by the Daleks and transferred his essence into a human body, he attempted to use the Eye of Harmony in the Doctor's TARDIS to transfer the Eighth Doctor's remaining five regenerations to him to restore himself to full life. This plan failed when the Doctor's ally Grace Holloway set the TARDIS into temporal orbit, severing the link between the Eye and the Master, returning the Doctor's regenerations to him and leaving the Master once again trapped in a dying body, which was subsequently lost when he fell into the Eye. (TV: Doctor Who)

During the Time War, the Time Lord Vassarian was captured by the Sontarans after he entered the Eternity Cage of his damaged Battle TARDIS as he started to regenerate, leaving him trapped in the moment of his own regeneration. The Sontarans were able to use the resulting artron energy released to power various temporal weapons, with the goal of opening a third front in the Time War, but the War Doctor was able to make telepathic contact with Vassarian as his mind was still active on some level. While technically frozen in time, Vassarian revealed that he had lost so much energy through the Sontarans' actions that he would be unable to regenerate, the War Doctor helping him end his life so that the Sontarans would be destroyed by the Daleks. (AUDIO: The Eternity Cage)

When the Third Doctor was drawn to a time period shortly after the Tenth Doctor had regenerated, the Tenth Doctor unknowingly drew on the energy of his past self to stabilise his own regeneration simply because the two Doctors were in close proximity to each other without either Doctor coming face-to-face. The Third Doctor described the experience as energy osmosis as the weaker incarnation took energy from the stronger, although he simply felt a brief fatigue and was then back to normal while the Tenth was fully restored. (PROSE: The Christmas Inversion)

In one instance, the Tenth Doctor used up one of his cycle's regenerations while still keeping the same face by channelling it into a a hand which was sliced off by the leader of the Sycorax (TV: The Christmas Invasion), allowing himself to heal his body, yet avoid physical changes. This energy remained active and was accidentally interacted with by a human, Donna Noble, triggering a two-way human-Time Lord meta-crisis which caused the hand to regenerate the missing body of its owner, creating a human-Time Lord copy of the Doctor and causing Donna to gain the mind of a Time Lord. However, the presence of two minds inhabiting Donna at once nearly destroyed her and the Doctor was forced to block all memories of him from her consciousness to stop her Time Lord half from remaining active. (TV: Journey's End, The Time of the Doctor)

Dead Time Lords could be revived with regenerative energy, though the amount of energy required to do this was very large and took a physical toll on the donor. Doing this, Melody Pond, in her third incarnation, burned up her ten remaining regenerations to save the Doctor's life and was hospitalised as a result. (TV: Let's Kill Hitler)

The Eleventh Doctor possessed enough regenerative energy to heal River Song's broken wrist, although she was angered at what she considered a waste of the energy. (TV: The Angels Take Manhattan)

The Twelfth Doctor attempted to donate a small portion of regenerative energy to the dying Davros to give him the strength to see a final sunrise, speculating that this would cost him an arm or a leg at some future date (or make him diminutive in stature), but Davros used this opportunity to siphon regenerative energy from the Doctor, and transmit it to all the Daleks across Skaro. While the Daleks grew stronger, Davros gained several more years of life. (TV: The Witch's Familiar) It remains unknown if this took up a complete regeneration's worth of energy. An illusionary version of the Twelfth Doctor attempted to temporarily cure his current blindness by transferring energy from his own future, noting that this may cost him the ability to regenerate in a worst-case scenario, but the long-term consequences of this were not important as this Doctor was only a digital recreation. (TV: Extremis)

The Twelfth Doctor also demonstrated the ability to release regenerative energy at will while unhurt, and without transferring it to anyone else, but simply allowing it to dissipate. This allowed him to essentially fake a regeneration without changing. It is unclear how much energy this used, or how it may have affected his ability to regenerate in the future. (TV: The Lie of the Land)

Control over regeneration
Generally, the regenerative process triggered itself when a Time Lord was too badly injured to survive; however, in some cases, Time Lords exercised control over the process. Romana I seemed to regenerate on a whim, (TV: Destiny of the Daleks) while Azmael deliberately attempted a thirteenth regeneration to end his life. (TV: The Twin Dilemma) When badly injured without actually sustaining regeneration-inducing injuries, the regenerative platelets in a Time Lord's blood could activate to accelerate their ability to heal, allowing them to recover from serious injuries more quickly than a human would, (AUDIO: The Bride of Peladon) although particularly serious yet non-fatal damage would require the Time Lord to enter a healing coma to recuperate. (TV: Planet of the Daleks, PROSE: Vanishing Point)

The degree of control that Time Lords had over their end appearance was unclear. The Rani stated, "Most Time Lords are at the mercy of fate after death." She desired the ability to definitively control the outcome of her appearance upon regeneration and attempted to learn how to do this by studying Koturian Phasing. Her attempt failed, however, because she was not in love with the man she was engaged to, which was the catalyst for Phasing. (PROSE: Something Borrowed) The Master had the ability to control the appearance of each of their regenerations. (PROSE: Harvest of Time) Melody Pond announced she was "focusing on a dress size" moments before her final regeneration. She also commented that she might "take down the age a bit, just to freak people out," although she did not elaborate whether this was a joke or a literal intention. (TV: Let's Kill Hitler) Romana I seemed adept enough at the process to custom design her new form during what seemed to be a voluntary regeneration. The Doctor criticised Romana II for taking on the form of another person, suggesting such things were not unheard of. (TV: Destiny of the Daleks) On an earlier occasion, Lord Roche was able to influence his next incarnation to be a double of the Third Doctor's appearance when he regenerated after being hit by a bus despite having only briefly seen the Doctor. (PROSE: The Suns of Caresh) When Glospin attempted to frame the First Doctor for the murder of Quences, he was able to deliberately regenerate into a duplicate of the Doctor after taking a genetic sample during a fight. (PROSE: Lungbarrow) When it came to changing sexes during regeneration, a Time Lord had no say in the matter, unless the regeneration was triggered willingly. (COMIC: The Fourth Wall) Lake observed his fellow clones sometimes exhibited control over aspects of their next bodies and realised he could influence his own regeneration by focusing on a specific appearance he desired as he was regenerating. He used this knowledge to influence one of his incarnations into the identical form of a man he murdered in order to steal his identity. (AUDIO: The Lady in the Lake)

In contrast to Romana, the Doctor did not seem to have much control over their post-regeneration appearance; after their fourth regeneration, the Fifth Doctor commented, "That's the trouble with regeneration, you never quite know what you're going to get." (TV: Castrovalva) The Ninth Doctor restated this sentiment immediately prior to their tenth regeneration. (TV: The Parting of the Ways) They also nearly always examined himself or asked about their appearance. (TV: The Power of the Daleks, Spearhead from Space, Robot, The Caves of Androzani, The Christmas Invasion, The End of Time, Twice Upon a Time) Both the Seventh (TV: Time and the Rani) and Tenth Doctor (TV: The Day of the Doctor) referred to regeneration as "a lottery". On two occasions, the Twelfth Doctor was able to expend regeneration energy at will without triggering a regeneration in himself; once when he attempted to give Davros enough energy to enjoy one final sunrise, speculating that the amount of energy he planned to donate would either cost him an arm or a leg or make him diminutive in stature in a later incarnation, though Davros exploited the compassionate action to steal a much larger quantity energy in order to renew the Daleks and extend his own lifespan to a further degree. (TV: The Witch's Familiar) He again seemed to expel some regeneration energy when faking his 'death' in order to test his companion. (TV: The Lie of the Land)

While skilled Time Lords could choose their new form with a voluntarily induced regeneration, the process could go horribly wrong and leave the Time Lord in a severely misshapen body. This problem might be exacerbated by the Time Lord immediately starting another regeneration instead of obtaining medical assistance, amplifying the defects in the regeneration. The end result of these abortive regenerations was inevitably a mutated monstrosity that could only be put out of its misery by complete disintegration. (PROSE: The Twin Dilemma)

In cases where Time Lords could not choose their new appearance, it appeared that their bodies still had some degree of natural "control" over the forms they would take upon regenerating. The Third and Seventh Doctors were once given glimpses of what their next incarnations would look like without actually meeting their future selves, when the Third was briefly possessed by the Nurazh and nearly regenerated before his injuries were healed (PROSE: The Touch of the Nurazh) and when the Seventh encountered Elizabeth Klein and learned about her relationship with the version of him that existed in the timeline where Ace died at Colditz. (AUDIO: Klein's Story) Despite these events occurring some time before their final deaths, their future selves witnessed here were both the incarnations they would regenerate into later. It was also suggested that the Time Lords have a subconscious control over their appearances; after the Twelfth Doctor's regeneration, he believed that he recognised his new face from somewhere (TV: Deep Breath), but it took some time (TV: The Girl Who Died) until he recognised it as the face of Caecilius (TV: The Fires of Pompeii), speculating that he 'chose' the face to remind himself that his purpose as the Doctor was to save people.

Some Time Lords were capable of momentarily regenerating, or partially regenerating. Though this could use up a lot of regenerative energy, it would give the Time Lord a new set of genes, allowing them to fool genetic sensors. The Seventh Doctor used this method on the planet Purgatory to fool the genetic scanner used by the Imperial Landsknechte. (PROSE: Original Sin) The First Doctor's appearance slightly changed as a result of him beginning to regenerate but refusing to allow the process to complete itself. (TV: Twice Upon a Time)

Forcing an individual to regenerate could be used as a form of punishment and in these cases, the new appearance could be chosen or influenced by the condemned. (TV: The War Games) Granting additional regeneration cycles (TV: The Five Doctors, Utopia, The Time of the Doctor) or removing any remaining regenerations (TV: The Ultimate Foe) was also possible. Rassilon's gauntlet could be used to remove regenerations, or at least force a Time Lord to repeatedly regenerate until their remaining lives were exhausted. (TV: Hell Bent)

The Sisterhood of Karn had elixirs that could trigger and control regeneration in a Time Lord who was dying and unable to regenerate normally or, in extreme cases, already dead. These elixirs could determine traits to be held by the new incarnation such as appearance, sex, apparent age, and personality. (TV: The Night of the Doctor)

With effort, Time Lords could resist regeneration, effectively committing suicide. (PROSE: The Power of the Daleks) did so after being shot, ostensibly to avoid becoming the Tenth Doctor's eternal prisoner. (TV: Last of the Time Lords) Similarly, the Fifth Doctor once threatened System with resisting regeneration to stop the device from learning the biological details of the act. (AUDIO: The Gathering) This was not always an option, however, as the Sixth Doctor noted fearfully that while his companion could die only once, he might repeatedly regenerate and live out all of his lives when the TARDIS stalled in space. (TV: Vengeance on Varos) Both the First and Twelfth Doctors at first refused to regenerate, but eventually relented, and went on to become their successive selves. (TV: Twice Upon a Time)

Rassilon had discovered a form of true immortality beyond the regenerations known to the Time Lords, but kept this a secret, believing it would be too dangerous to share. He went to the extreme of entombing the secret with himself in the Death Zone of Gallifrey and petrifying all those who came to inquire of the method. Borusa described Rassilon's immortality as a "timeless, perpetual, bodily regeneration". (TV: The Five Doctors)

After deciding to help the Doctor, the Missy incarnation of the Master stabbed her past self in such a way that the wound was "precise", forcing the Master to regenerate but giving him a period of time before the process was triggered. (TV: The Doctor Falls)

State of grace
According to the Twelfth Doctor, there is a period of time that he calls "a state of grace" when a dying Time Lord can resist regeneration for a period of time without consequence where they are restored to full strength. During this period of time, the Time Lord must choose whether or not they will ultimately regenerate or they will grow weaker as time passes by. At this period's end, the Time Lord must regenerate or die permanently. (TV: Twice Upon a Time)

While a state of grace could be voluntary, some forms of death could induce a similar state unless they received aid. When the Third Doctor contracted radiation poisoning on Metebelis III, (TV: Planet of the Spiders) he spent ten years drifting in his current form in the TARDIS before he finally died upon returning to UNIT (PROSE: Love and War), only regenerating with the aid of K'anpo Rimpoche. (TV: Planet of the Spiders)

The Fifth Doctor was able to hold off his regeneration from Spectrox toxaemia for several hours while trying to retrieve the bats' milk needed for the antidote but finally accepted the need to regenerate when he was only able to acquire enough antidote for his companion, Peri. (TV: The Caves of Androzani)

The Ninth Doctor momentarily delayed his impending regeneration to explain what was happening to Rose Tyler. (TV: The Parting of the Ways) After subjecting his body to a lethal amount of radiation, the Tenth Doctor delayed the regenerative process long enough to revisit each of his former companions from his own incarnation and his past selves as well (TV: Death of the Doctor) before he finally regenerated, which severely damaged the TARDIS. A few minutes before the regeneration began, his "state of grace" ended causing the Tenth Doctor to experience excruciating pain, resulting in him collapsing to the ground, barely possessing the strength to reach his TARDIS. (TV: The End of Time) When the Eleventh Doctor was granted a new regeneration cycle on Trenzalore, he was able to use the energy from the resulting "reset" to regress his body to a younger state and hold back the regenerative process for a time. During such, he phoned his personal future to ensure Clara Oswald stayed with his future self, (TV: Deep Breath) and later said goodbye to his companion before finally undergoing a full regeneration. (TV: The Time of the Doctor)

The First Doctor, fearing the change, held off his own regeneration for several hours. However, unlike his future incarnations, the First Doctor face did not stay the same, becoming "mixed up" while in the state of grace. When the First Doctor regenerated, he passed out as his state of grace ceased. (TV: Twice Upon a Time) However, his regeneration was comparatively peaceful than when his future selves regenerated after halting the process (TV: The Tenth Planet)

When severely injured by a Cyberman, the Twelfth Doctor was able to resist regeneration for several weeks despite struggling to walk. He further resisted it despite numerous Cybermen mortally wounding him, buying himself time to ignite Floor 0507. However, he struggled to hold it off when revived in the TARDIS, nearly regenerating completely. (TV: The Doctor Falls) The Twelfth Doctor encountered the First Doctor at this time, who was also in a state of grace, holding back his regeneration as he feared regenerating. Throughout this time, the Twelfth Doctor steadily grew weaker, nearly collapsing at one point while at Villengard and having to sit down for a few minutes to recover. After the adventure, both Doctors chose to regenerate. While holding his regeneration back a little longer to make an advisory speech to his future incarnation, the Twelfth Doctor continued to grow weaker and collapsed to the floor just before his regeneration occurred though he managed to stand up once more by leaning on the TARDIS console to help him to his feet. Similar to the Tenth and Eleventh Doctors' regenerations, the Twelfth Doctor regenerated explosively, causing the control room in the TARDIS to explode. (TV: Twice Upon a Time)

Regenerative difficulty
While most regenerations seemed to cause moments of mental instability, with temporary amnesia often noted, some offered particularly profound instances of physical peril. The Fifth Doctor feared that his regeneration was "failing" when he found himself reverting to previous personas, and could only be righted with the use of the TARDIS Zero Room. Ambient complexity could also contribute to the failure of a regeneration. (TV: Castrovalva) The Eighth Doctor claimed that anaesthesia had "nearly destroyed the regenerative process" during his seventh regeneration as an explanation for the particularly severe amnesia he suffered afterwards. (TV: Doctor Who) During the Tenth Doctor's post-regenerative state, he suffered an arrest in one of his hearts and began to exhale regenerative energy when Rose Tyler revived him too early. After this, he said that he was having a neural implosion, and slipped into a coma-like state for most of the day. (TV: The Christmas Invasion) When the Tenth Doctor underwent his own regeneration, the process was exceptionally violent and destructive to the TARDIS. (TV: The End of Time) When the Eleventh Doctor regenerated, (TV: The Time of the Doctor) he was so scared at the prospect of what he would become in his next body that he called his own future to ensure that his current companion would stay with his future self to help him through it, not trusting that his next incarnation would be able to hold on to who he was as the Doctor without her help. (TV: Deep Breath) The Twelfth Doctor's regeneration, much like the Tenth's, damaged the TARDIS. (TV: Twice Upon a Time)

Regeneration, especially later ones, could be painful. Melody Pond screamed during one regeneration, (TV: Let's Kill Hitler) as did the Master during his transition into. (TV: Utopia) The Ninth Doctor winced in pain as his regenerative energy spiked, (TV: The Parting of the Ways) the Tenth Doctor also appeared to grimace in pain during the process, and the Eleventh Doctor screamed as he emerged from the regeneration. (TV: The End of Time) Once, Sarah Jane Smith asked the Eleventh Doctor if his last regeneration had hurt. After trying to deflect the question, he quickly said, "It always hurts," before, in the same breath, continuing with the task at hand. (TV: Death of the Doctor) The Eighth Doctor's regeneration was painful due to the circumstances, but he wanted it so. (TV: The Night of the Doctor)

Reversal
There were many ways to reverse a regeneration. One way involved the sacrifice of another, causing the regeneration to reverse. One example of this was when the Third Doctor had an encounter with the Nurazh. As the Doctor battled the Nurazh's main host, the two fell off a building, killing the Doctor. As the Third Doctor (nearly) regenerated into the Fourth, the Nurazh possessed the Time Lord's body; however, it found itself unable to cope with the two Time Lord minds within the body and it soon perished, restoring the Doctor to his previous incarnation in the process. (PROSE: The Touch of the Nurazh)

When trapped in a dimensionally-unstable pocket universe created and controlled by Iam and the Rani, the Sixth Doctor's morphic print was destabilised, causing him to unwillingly and painfully regress back through his previous selves as his body sought a stable morphic print. He was forced to rely on the stabilising atmosphere of the TARDIS and a personal morphic stabiliser he designed to operate in this realm until it was returned to the real universe. (PROSE: State of Change)

Some Time Lords of the first rank attempted retro-regeneration, reverting from their current incarnation back into a prior body, but this procedure was relatively rare, to the extent that the Sixth Doctor was unable to recall any examples of it. (PROSE: State of Change) The Tenth Doctor once reverted into the Ninth Doctor when an artefact reversed his local time field. Moments later, he changed back, but wondered if he got back the regeneration he wasted. (COMIC: The Fountains of Forever)

On rare occasions, it was possible for Time Lords to deliberately allow themselves to mentally regress back to the personas of previous selves while remaining the same incarnation physically. While newly regenerated Time Lords could briefly act like past selves as their new bodies and personas stabilised, (TV: Castrovalva) when he was trapped in Iam's unstable realm, the Sixth Doctor was able to use his current morphic instability to allow the Third Doctor's persona to take control, allowing him to draw on his past self's skill for hand-to-hand combat. (PROSE: State of Change) On another occasion, the Seventh Doctor used the TARDIS telepathic circuits to bring the Third Doctor to the fore so that he could use his past self's skill with technology to disarm a dangerous bomb, although his control sometimes slipped as his current self tried to assert itself, resulting in the Third Doctor referring to his current associates by the names of his own companions. (PROSE: Timewyrm: Genesys)

Regenerative dissonance
Considered as a rare mental illness, Time Lords might not lose their past personas when they regenerate. As a result, these Time Lords acquired new physical bodies, but the past personas remained conscious and active in their minds, rather than simply retreating into the Time Lord's subconscious. These past incarnations thrived and accumulated inside the current incarnation's head, fighting over control of the physical body. The only known Time Lord who had this illness was The Eleven. (AUDIO: The Eleven) Immediately after regeneration, the previous incarnations would be mentally weaker and more disorientated, but they would eventually asset themselves and resume the Eleven's warped sense of mental balance. (AUDIO: The Crucible of Souls) Other Time Lords were known to suffer from this illness, but they often committed suicide as they couldn't cope with the voices. The Time Lord who lasted the longest with this condition after the Eleven died after shooting out both his hearts with a staser in his eighth body. (AUDIO: World of Damnation)

Avoiding change
A Time Lord could avert the change of appearance and personality caused by regeneration by focusing the regenerative energies into a "bio-matching receptacle", as the Tenth Doctor did with his own severed hand. The hand siphoned off the excess energy that would have changed his appearance while the Doctor used just enough to heal himself (TV: Journey's End) from the injury sustained from a Dalek Gunstick. (TV: The Stolen Earth) This resulted in the appendage storing enough residual energy to grow an identical Time Lord when it came in contact with Donna Noble soon after. (TV: Journey's End) As a result, the Doctor used up a full regeneration (his eleventh of twelve overall), (TV: The Time of the Doctor) but did not change his incarnation, allowing him to avoid the usual post-regenerative confusion and disorientation experienced in the past. (TV: Journey's End)

During the time when he controlled the Source of Traken, (TV: The Keeper of Traken) the Master was able to use the Source to heal his injuries in place of regenerating, noting that this process was far smoother than regeneration as it avoided sacrificing the healthy tissue in the process, although this method eventually proved short-term when he expended the last of the Source. (PROSE: The Quantum Archangel)

The regeneration process could also be delayed to allow healing. The Second Doctor was shot in the head when confronted by guards on Skybase, causing damage to his skull and frontal lobe; and the subsequent fall broke his nose, jaw, right femur, and collarbone, along with some spine damage. He began to regenerate, but an injection of Shiner DNA delayed the regeneration and kept him alive long enough for his body to go into a six-month healing coma to recover on its own, although he was briefly certain that he had regenerated when he woke up. (PROSE: The Indestructible Man) When the Seventh Doctor deliberately affected himself with light wave sickness to save the Spiridons from the Daleks, he briefly believed that he was going to regenerate until he retreated to the TARDIS, his body spending some time fighting between its cellular paralysis and natural desire to regenerate until it stabilised in his current self. (AUDIO: Return of the Daleks) The Twelfth Doctor managed to delay regeneration after being electrocuted by a Mondasian Cybermen, attempting to allow himself to recover without the need for such despite struggling to walk. However, his subsequent injuries in the Battle of Floor 0507 forced him into the "state of grace" where he had to decide whether he would regenerate or die. (TV: The Doctor Falls) The First Doctor also experienced such, delaying regeneration in fear of what the change would bring. (TV: The Tenth Planet) Both eventually decided to regenerate though, each accepting their futures. (TV: Twice Upon a Time)

When his second heart was extracted by Sabbath and placed in Sabbath himself, the stolen heart created a link between the Eighth Doctor and Sabbath that rendered the Doctor essentially immortal; as his second heart was still beating in Sabbath's chest, the Doctor could survive normally fatal injuries, such as having his chest crushed by sandbags or being stabbed in his remaining heart, without changing, although he would go into a near-death state until his body could heal. However, it was unspecified if there were any limits to this connection- the Doctor only needed to use it when he and Sabbath were on the same planet at the same time in the same city- and the connection was lost when Sabbath tore the Doctor's heart out of himself. (PROSE: The Adventuress of Henrietta Street, Camera Obscura)

Cross-species transformations
I.M. Foreman, a Gallifreyan (but not a Time Lord), absorbed the DNA around him and underwent indescribable changes as a result of mutations, transcending sex, species and even physical existence itself. (PROSE: Interference - Book One, Interference - Book Two) Romana, prior to her regeneration into her second incarnation, appeared to have taken on a humanoid blue-skinned form. (TV: Destiny of the Daleks) However, one account held that the TARDIS itself, rather than Romana, adopted this shape. (PROSE: The Lying Old Witch in the Wardrobe) Former Cardinal Zero regenerated into the form of the avian species whose planet he had taken residence on. (AUDIO: Spring)

Aborted regeneration
Occasionally, a regeneration would fail and the process would abort. Though the Time Lord would have regenerated, they would be severely deformed. Though Time Lord technology could treat this, on some occasions the damage would be too severe to fix.

After being shot by the War Lords, the War Chief was barely able to survive. While being returned to the War Lords' planet, his body attempted to regenerate. Due to the massive injuries and the lack of medical care, this regeneration aborted. This resulted in two conjoined individual bodies, poorly fused together, and also apparently compromised his ability to regenerate ever again, preventing him from simply regenerating once more to solve the problem. (PROSE: Timewyrm: Exodus)

Iterative regeneration
During the Last Great Time War, Rassilon experimented over other Time Lords, retro-evolving their timelines and connecting them to the time vortex, in order to build a possibility engine - a machine to question about decisions to make during the conflict. A side effect of the process on the so-called Interstitials was the trapping in a loop of an iterative regenerative cycle, which caused their appearance to enter a state of constant flux among their different incarnations. (PROSE: Engines of War)

Attitude toward regeneration
As noted above, regeneration was not guaranteed. The Doctor on numerous occasions believed he was at risk of actually dying. Even with regeneration a possibility, the Doctor came to feel such a change as being a "death". In recollecting the events surrounding the Master's attempt to steal the Eye of Harmony, the Eighth Doctor referred to his incarnations as "lives". (TV: Doctor Who) The Doctor's third, (TV: Planet of the Spiders) fourth, (TV: Logopolis) ninth, (TV: The Parting of the Ways) and tenth incarnations (TV: The End of Time) referred to their regenerations as the end of their life. The Twelfth Doctor also referred to regeneration (though not his own) as the same as death, but also stated that to Time Lords, death was simply "man flu". (TV: Hell Bent) When faced with regeneration himself, however, this Doctor truly valued his life, resisting the process as he didn't want to change. (TV: The Doctor Falls) When he finally accepted the inevitable, he spoke out loud instructions for his next incarnation to follow as if actually speaking with them. The First Doctor held similar reservations, disliking the change he would undergo. After seeing what his future would bring though, he felt more comfortable, embracing the change as he lost consciousness. (TV: Twice Upon a Time)

In fact, the Doctor seemed to regard their previous incarnations as different individuals, capable of interacting and working with each other (TV: The Three Doctors, The Five Doctors, The Two Doctors, Time Crash, The Day of the Doctor, Twice Upon a Time) However, they still regarded their otherselves as them, to the extent that their seventh incarnation became ashamed of the actions of his sixth self, going through a period of keeping his memory of his previous self locked up in his mind (PROSE: Head Games) until he accepted that he was the Doctor in all of his incarnations, and forgave his past self's sins after he nearly died after being shot by an arrow. (PROSE: The Room With No Doors) Younger or older Doctors could also disapprove of their other selves, such as the Fifth Doctor being horrified when he witnessed what the Seventh Doctor was capable of, (PROSE: Cold Fusion) the Eighth Doctor being ashamed at the manipulations of his past self, (AUDIO: The Resurrection of Mars) the Third Doctor regarding his immediate predecessor as a distraction who would actually make him less effective, (TV: The Three Doctors) the Tenth Doctor expressing a rather low opinion of his previous self, considering him to be violent due to being "born in battle", (TV: Journey's End) and the Twelfth Doctor saying that his first incarnation was "rude to people" in his early days. (TV: Hell Bent) The Missy and Harold Saxon incarnations of the Master grew to outright despise each other, despite having grown attracted to each other when working together, due to their juxtaposed view of the Doctor, to the point that they actually killed each other. (TV: The Doctor Falls)

At the same time, meeting other Doctors could allow other incarnations the chance to re-evaluate their opinion of themselves, such as the Eighth Doctor coming to admire the Sixth Doctor where his seventh had feared his potential, (PROSE: The Eight Doctors) and the Tenth and Eleventh Doctors recognising that the War Doctor was a true Doctor despite denying him for years. Their acceptance of the War Doctor was due to their discovery that, far from destroying Gallifrey, he had helped them and all their other selves place Gallifrey in a pocket universe to protect it. It was also revealed that the Eleventh Doctor remembered that incarnation destroying the Time Lords because the time streams of his other incarnations were out of sync with his own, resulting in all his previous selves losing their memories of the Time War's true ending, as happened to the War Doctor during his regeneration. (TV: The Day of the Doctor)

The Twelfth Doctor was also rather fond of the First Doctor, amused at his old ways in how he still called the TARDIS "the Ship", and how he looked wearing the sonic sunglasses. However, he was incredibly embarrassed by his original incarnation's frequent sexist remarks. The First Doctor was dismayed at learning the Twelfth Doctor played the electric guitar, and was disappointed to learn he no longer had women cleaning the TARDIS for him. He also criticised the Twelfth Doctor's over-reliance on technology, over his own intellect, and his need to always boast about his plans. As a whole, the First Doctor was horrified to learn he would eventually become a "Doctor of War". However, he grew to admire his future self, believing their actions were for the greater good rather than malicious purposes as he initially believed. (TV: Twice Upon a Time)

Most other Time Lords never expressed any strong opinions about their other incarnations as they had never met their other selves. However, when the Time Lord Straxus learned that he would become the insane Kotris in his next incarnation, he was horrified at his next self, proclaiming that Kotris was a psychopath, although Kotris claimed that his insanity was the result of Straxus' insanity and self-loathing. However, despite his disgust at Kotris's actions, Straxus only made a few half-hearted efforts to kill himself to avoid becoming Kotris which were prevented by a drone Kotris had sent, until the final confrontation between the two incarnations culminated in Straxus being exterminated as even the Daleks were disgusted with his selfishness. (AUDIO: X and the Daleks) When the Master made contact with the Cult of the Heretic and was offered an alliance with them if he killed one of his past selves- with the promise that the Cult would use the Anomaly Cage to prevent him being wiped out by the paradox- the Master laughed as he dismissed his past incarnations as foolish. Although he later claimed that he had targeted his past self at a point when he knew that the younger Master would survive, the two Masters found it difficult to cooperate, as the younger Master was more serious and dedicated to ensuring victory where the future Master's new lease on life had left him more inclined to make various bad jokes as he taunted his enemies. The older Master noted that the Cult's plans to remake the universe had been inspired by the beliefs of the renegade Time Lord known as the Heretic, whose belief that the universe was sick, led him to perceive regeneration as the only cure for this 'illness'. (AUDIO: The Two Masters)

Time Lords such as the War Chief were unconcerned about wasting regenerations, or Romana I, who regenerated with no apparent need in order to assume an appearance she liked, (TV: Destiny of the Daleks, PROSE: The Shadows of Avalon) while others such as the Doctor warned not to waste them. (PROSE: Invasion of the Cat-People) Iris Wildthyme once confided in Samantha Jones that regeneration was treated on Gallifrey the same way sex was on Earth. (PROSE: The Scarlet Empress)

In general, the Doctor avoided discussing regeneration with their companions unless someone else brought it up first, (TV: Planet of the Spiders) but explained the process in the aftermath. They were particularly open about the process in their eighth incarnation, telling companion Charley Pollard about regeneration and their past faces, noting at one point that he considered regeneration superior to the straightforward immortality of the ruthless Sebastian Grayle, as regeneration allowed him to change and develop as time went on where Grayle was stuck with one point of view and no real way to change. (AUDIO: Seasons of Fear) Despite this, the Doctor's attitude towards regeneration seemed to change during their later incarnations, considering it more like true death. In their ninth incarnation, the Doctor bade farewell to his companion, ("I'm not gonna see you again. Not like this.") even though he was not actually dying. (TV: The Parting of the Ways) The Doctor's tenth incarnation was concerned about a prediction made regarding his own impending regeneration, saying, "Even if I change, it feels like dying. Everything I am dies. Some new man goes sauntering away...and I'm dead." (TV: The End of Time) Their tenth incarnation also compared regeneration to a lottery, in terms of what he becomes as a result of it; when meeting his immediate successor for the first time, he joked that his eleventh incarnation's bigger sonic screwdriver was "compensating". (TV: The Day of the Doctor) Following his regeneration into his seventh incarnation, the Doctor's memories of his sixth self's persona came to resent the current Doctor, accusing the current Doctor of "murdering" him (PROSE: Head Games) before the Doctor came to accept that he was the Doctor in all his lives and forgive the sins of his previous self. (PROSE: The Room With No Doors) The War Doctor, however, accepted the start of his regeneration (ultimately into the Ninth Doctor), remarking that his old body was "wearing a bit thin" and even joking about his hopes for getting less-prominent ears. (TV: The Day of the Doctor) After receiving his new regeneration cycle, the Eleventh Doctor appeared relatively comfortable about his imminent regeneration, reflecting that everyone changed throughout their lives and the important thing was to remember who you had been, (TV: The Time of the Doctor) although his fear about the scale of the change he was about to experience prompted him to call his current companion in their personal future to ask her to stay with his next incarnation and help him through the transition to his new body. (TV: Deep Breath) The Twelfth Doctor later recalled that the end of the First Doctor's life at Snowcap was "the place where [he] died", comparing it to Clara Oswald's limited memories of her 'splinters' by describing it as something so huge and terrible that the mind had to block it out in the aftermath. (COMIC: Blood and Ice)

Despite his own attitude toward regeneration, both Harriet Jones (TV: The Christmas Invasion) and Sarah Jane Smith (TV: Death of the Doctor) felt the same way about the Doctor throughout their incarnations. Harriet called the Tenth Doctor "absolutely the same man", still believing in this despite the Doctor threatening to destroy her government after she ordered Torchwood to blow up the Sycorax spaceship. (TV: The Christmas Invasion, The Stolen Earth) While the Brigadier noted that one Doctor was more than enough to deal with at any time, (TV: The Three Doctors) he nevertheless confidently proclaimed that all of the Doctors were remarkable chaps, willing to work with whatever Doctor answered his calls for help even if he acknowledged that he knew certain Doctors better than others. (PROSE: The Shadow in the Glass) Despite the importance of regeneration, the Doctor often failed to mention it to their companions, with the result that Ben, Polly, (TV: The Tenth Planet, The Power of the Daleks) Peri (TV: The Caves of Androzani, The Twin Dilemma) and Rose (TV: The Parting of the Ways, Children in Need Special) initially didn't believe that the new Doctor was the same person after their regeneration was complete. Even those companions who had been informed about the process in advance—Sarah Jane Smith (TV: Planet of the Spiders), Adric (TV: Logopolis), Melanie Bush (TV: Time and the Rani), and Clara Oswald (TV: The Time of the Doctor)—took a while to accept the new Doctor.

Even if a Time Lord's body was damaged to the point that regeneration became impossible, their cells would continue attempting to do so, making the actual process of death very slow; it took each copy of the Twelfth Doctor imprisoned in his Confession Dial "about a day and a half" to die after being attacked by the Veil. (TV: Heaven Sent) It can also be assumed that this slow death could only occur if the Time Lord was given no opportunity to regenerate before the killing blow was dealt, as the Teselecta duplicate of the Eleventh Doctor at Lake Silencio appeared to die instantly when shot mid-regeneration (TV: The Impossible Astronaut), while the Twelfth Doctor's body did not begin its attempt at regeneration until after he had been attacked by the Veil. (TV: Heaven Sent)

In some cases, it was possible for the regenerative process to be restarted by another Time Lord if it failed, such as when K'anpo Rimpoche gave the Third Doctor's body "a little push" to initiate the regeneration process after he was exposed to the radiation of the Metebelis crystals, although he warned that the new Doctor would be somewhat shaken up as a result of this method. (TV: Planet of the Spiders)

After deciding to help the Doctor against the Mondasian Cybermen, the Missy incarnation of the Master was shown to see her past self as still being her, stating that she'd loved being him and the feeling of all that he was. However, due to her genuine desire to change, Missy mortally wounded her past self to force his regeneration into herself, appearing to see it as necessary to ensure the Master became Missy. (TV: The Doctor Falls)

Regeneration in other species
On several occasions, individuals of other species (or even artificial life) were capable of regeneration. In most cases, this ability was copied from the Time Lords.

The Kastrians, a silicon-based life form, were able to undergo an explicit process of regeneration, by absorbing radiation from the environment and/or in a dedicated "regeneration chamber" set with their specific genetic code. This form of regeneration was explicitly shown to allow a change in gender. (TV: The Hand of Fear)

Kate Yates regenerated her hair when her Dalek Factor was activated after being hit by a car. (PROSE: I Am a Dalek)

The crew of the Minyans' ship, the P7E, could regenerate indefinitely. Over time, they wearied of life. (TV: Underworld)

Mawdryn and his followers, who had stolen the Time Lords' regeneration technology, also had a great number of incarnations, though they had no control over when it would happen and what form, often grotesque, they would change into. Consequently, they also longed for death, making their mutations a kind of de facto punishment by the Time Lords for stealing their technology. (TV: Mawdryn Undead)

Chris Cwej was regenerated by force to survive radiation poisoning. (PROSE: Tears of the Oracle)

K9 Mark I regenerated by use of a regeneration unit after self-destructing to defeat a group of Jixen Warriors. (TV: Regeneration) He later regenerated again after exhausting his power core to defeat the Trojan. (TV: The Eclipse of the Korven)

Why regeneration?
Regeneration was introduced to the mythos of Doctor Who to solve a practical staffing problem: the production team needed to find a way to exit William Hartnell but still keep the show running.

The original idea for this replacement came from producer John Wiles and script editor Donald Tosh. They proposed to write out Hartnell during The Celestial Toymaker, a serial they commissioned and prepped, but ultimately didn't produce. Their notion was that the Celestial Toymaker would make the Hartnell Doctor disappear, but when the Doctor re-appeared he would magically be another actor entirely. (REF: The First Doctor Handbook, The Second Doctor Handbook) Though not at all a regenerative process as the term has since come to be understood, Wiles and Tosh do at least get some credit for being the first people to moot the possibility of carrying on the show with a new lead —and for the necessity of finding a narrative explanation for this switch. Though this is taken for granted today, this was an important milestone on the way to regeneration. Doctor Who could just as easily gone down the route of another 1960s show,, where a main character was simply recast without narrative explanation.

However, Wiles and Tosh were ultimately unsuccessful in their bid to replace Hartnell, due to resistance from BBC Head of Serials, Gerald Savory. (REF: The Second Doctor Handbook) This failure was a part of the reason Wiles resigned relatively soon after taking over the show from Verity Lambert. (REF: The First Doctor Handbook)

His successor, Innes Lloyd, was better able to negotiate Hartnell's departure, in part because the climate within the BBC hierarchy had changed with Shaun Sutton's management elevation. (REF: The First Doctor Handbook) Still, it is uncertain who, precisely, came up with the idea of regeneration-as-biologic-process, rather than the mystical solution Wiles had earlier mooted. Howe, Stammers and Walker believe "the likelihood is that it emerged in discussion between Lloyd and his story editor Gerry Davis" — along with additional input from Shaun Sutton, and Kit Pedler. (REF: The Second Doctor Handbook)

Narrative origins
The earliest-known production office-generated document on the subject describes it thus:"The metaphysical change which takes place every 500 or so years is a horrifying experience — an experience in which he re-lives some of the most unendurable moments of his long life, including the galactic war [which was believed, at this time, to have been the cause of the Doctor and Susan's departure from their home planet]. It is as if he has had the LSD drug and instead of experiencing the kicks, he has the hell and dank horror which can be its effect."

- 1966 production note, entitled "The New Doctor Who", The Second Doctor Handbook p.24

Initially, the concept wasn't called "regeneration" at all, but rather "renewal". In fact, the term, so familiar to Doctor Who fans today, didn't appear until the Doctor's third regeneration, first seen by fans in 1974's Planet of the Spiders. Since The War Games, Troughton's final story, merely had the Time Lords suggesting that they would "change [his] appearance", the only explanation of regeneration — for the show's first twelve years — was found in a cryptic exchange in part one of The Power of the Daleks:


 * Ben: Now look, the Doctor always wore this. So if you're him, it should fit now, shouldn't it?
 * Ben grabs the Doctor's hand and slips the signet ring on. But the ring, too big for the new Doctor's finger, falls to the TARDIS floor.
 * Ben: There - that settles it.
 * Doctor: I'd like to see a butterfly fit into a chrysalis case after it's spread its wings.
 * Polly: Then you did change!
 * Doctor: Life depends on change -- and renewal.
 * Ben: (sarcastically) Oh so that's it. You've been renewed have ya?
 * Doctor: (considering the notion seriously) I've been renewed, have I? That's it, I've been renewed! It's part of the TARDIS. Without it, I couldn't survive.

Although the Second Doctor's last claim of a connection between the TARDIS and regeneration has never been explored in detail, it is heavily suggested by later regeneration stories. Notably, the Third and Fourth Doctor's highest initial priority is returning to the TARDIS; (TV: Spearhead from Space, Robot) the Fifth Doctor desperately needs the TARDIS' Zero Room to assist with his regeneration; (TV: Castrovalva) and the Eighth Doctor's post-regeneration amnesia is instantly resolved when Chang Lee opens the TARDIS' Eye of Harmony. (TV: Doctor Who) The connection between man and vehicle was made explicitly clear by the visual effects in The Parting of the Ways. There, the effect used for regenerative energy was the same as the energy that emanated from and was returned to, the heart of the TARDIS.

Tweaking regeneration
Beginning with the regeneration that resulted in the Fourth Doctor, each successive regeneration reveals a bit more about the mystery of the act.

Planet of the Spiders shows viewers that one Time Lord can help another by giving the process "a little push". This act of "gifting" regenerative energy is later expanded upon in Mawdryn Undead and Let's Kill Hitler. Both these stories take Cho Je's "push" one step further by suggesting that regenerations can be outright gifted from one being to another.

The "Cho Je push" is also tweaked a bit for the Doctor's fourth regeneration. In Logopolis the audience is introduced to a kind of "mid-regeneration Doctor", a being called "the Watcher" who exists between the Fourth and Fifth Doctors. He then merges with the dying Fourth Doctor to start the regenerative process, and thus become the Fifth.

The notion that there is an existence for the Doctor within the act of regeneration is again mooted by the audio story Winter, which takes place almost entirely in that interim between incarnations. In winter, the Doctor again merges with the Watcher to complete the transition into his next incarnation, though on this occasion the merging takes place inside the Doctor's mind between psychic recreations of both the Doctor and the echo of his future represented by the Watcher. Another "intra-regenerational" version of the Doctor is seen in The Trial of a Time Lord. The story's chief antagonist is implied to be the Doctor between his twelfth and thirteenth lives. A variation of this was seen in The Brink of Death, which depicts the Sixth Doctor in his mind at the moment of regeneration, including a brief moment where the Seventh Doctor 'speaks' to the Sixth before their voices merge as they state "It's far from being all over...", marking a successful regeneration.

Another novelty of the fourth regeneration is the introduction of the idea that a regeneration can "fail", resulting in the Doctor's death. (TV: Castrovalva) But if the fourth regeneration focuses on a physical crisis, the next three surely stress the mental hardships of the act. The fifth regeneration leads to a kind of mania never before experienced by the Doctor. It even shakes loose some criminal tendencies. (TV: The Twin Dilemma) The next two regenerations cause temporary amnesia. (TV: Time and the Rani, Doctor Who) This condition is particularly profound in the newly arrived Eighth Doctor, who completely forgets all of his past history for a number of hours. Additionally, complications like amnesia can be brought on by anaesthesia, which holds chemical agents that interfere with regeneration. (TV: Doctor Who) This regeneration also brings forth the notion that the Doctor actually dies prior to the metamorphosis of regeneration. The idea that the Doctor dies, even if briefly, is something that the Tenth Doctor later explains to Wilfred Mott in the first part of The End of Time.

The tenth regeneration, whose after-effects are documented in The Christmas Invasion, introduces the notion that the regenerative cycle lasts for fifteen hours. Within that window, the Doctor can lose body parts and yet re-grow them as he does with a hand he loses in battle with a Sycorax. Both Invasion and the preceding mini-episode also add another wrinkle to the mythos of regeneration. They show that the Doctor needs to expel regenerative energy in the aftermath of a change — something seen again in The Eleventh Hour.

The Doctor's twelfth regeneration is shown to be tangibly explosive, something that hadn't been explored by any previous BBC Wales — or, for that matter, any — regeneration. That is, regenerative energy is depicted as being able to physically damage things. By the end of the cycle, the Doctor's TARDIS is itself in need of a "regeneration." It is implied that this explosive regeneration is due to the Doctor delaying it while he travelled to see all his former companions for an unknown period of time, thus allowing that regenerative energy to build up. (TV: The End of Time, The Eleventh Hour, Death of the Doctor)

Aspects of both the ninth and tenth regenerations are invested in River Song's second regeneration, as seen in Let's Kill Hitler. River Song practically begs to be shot by Nazi soldiers immediately after regeneration so that she can re-trigger her explosive regenerative energy and hurt them. The Hitler regeneration also definitively proves that skin colour can change through regeneration — though this had actually been practically settled long before by the "blue option" seen in Romana's Destiny of the Daleks regeneration.

In Twice Upon a Time, the story featured what the Twelfth Doctor called "a state of grace" where the regenerating Time Lord is restored to full health for a period of time, but grows steadily weaker and must decide whether or not to regenerate once its over. This "state of grace" allowed an explanation for the Tenth Doctor's ability to delay regeneration to visit all of his former companions (TV: The End of Time, Death of the Doctor) and allowed for a story where the First Doctor and the Twelfth Doctor shared an adventure together that would help them determine whether or not they would regenerate or die.

Non-narrative explanations
Because of a relative lack of narrative explanation about regeneration, some writers of non-fictional or reference books about Doctor Who have tried to fill in the gap. One theory from such a source is that regeneration is caused by a "nanomolecular virus" that rebuilt the body much like the "self-replicating biogenic molecules". (REF: The Gallifrey Chronicles) This theory has not been repeated elsewhere, however.

That regenerative look
Each new regeneration was also radically different from the previous one, even in terms of the visual effects used to represent the moment of regeneration.

Do you remember the first time?
The very first regeneration was devised and executed by vision mixer Shirley Coward, who had rather unexpectedly come up with a method of achieving the effect electronically.

The original plan of the production team was simply that William Hartnell would fall to the floor at the end of The Tenth Planet and pull his cape over his face. Troughton would then appear at the top of The Power of the Daleks, retracting the cloak. Coward's then-innovative vision mix necessitated that Troughton be hastily contracted for The Tenth Planet, part four. The series' first regeneration sequence was then duly recorded on 8 October 1966, with the cliffhanger resolution filmed two weeks later on 22 October. (REF: The Second Doctor Handbook)

Later regenerations
Each subsequent regeneration was then filmed in a variety of different ways, as dictated by the director on that particular episode. Indeed, no two regenerations were particularly similar until the Russell T Davies era.

Only BBC Wales Doctor Who attempted to standardise the way regeneration looked. With The Parting of the Ways came what is now the standard "golden glow explosion" (although the colour of the explosion is fiery orange in The Parting of the Ways and is milky white in Utopia). The subsequent Children in Need Special established that there was residual "regeneration energy" after a transformation which had to be expelled through the mouth. This was seen again in The Christmas Invasion, The Eleventh Hour and The Doctor's Daughter — though the latter narrative never made quite clear that Jenny actually regenerated.

This visual standardisation has allowed narratives to play around with regeneration. The mere presence of "regeneration energy" can now be used to heighten dramatic tension. This visual shortcut, unavailable to production teams in the classic era, has been a particular favourite of Steven Moffat, who used the "golden glow" liberally throughout the 2011 series; in fact, unlike in the Russell T Davies era, in which nearly every regeneration had subtle differences, every Moffat era regeneration until TV: The Time of the Doctor is nearly identical. Several of the 2011 episodes used that VFX in a way that wordlessly suggested regeneration.

The standardised visual effects style carried into Staz Johnson's art for the 2017 comic story Doorway to Hell, which featured pre-Parting of the Ways and Utopia starting to regenerate. Colourist James Offredi also coloured the glow in golden shades similar to what was shown in the Russell T Davies and Steven Moffat eras.

Fan fare
Fans have long speculated as to whether the Doctor could change sex or skin colour as a result of a regeneration. They've also long speculated on the number of times that a Time Lord can regenerate since both Doctor Who and The Sarah Jane Adventures have given different impressions on separate occasions.

Gender
It had long been "fanon" that regeneration can cause a change of gender in Time Lords. This theory was proven correct with the regenerations of several characters in the DWU, including the Doctor, whose thirteenth incarnation was a woman. Eldrad already displayed this change in The Hand of Fear and even stating that his species' regeneration process was the same as the one used by the Time Lords.

Female versions of the Doctor previously appeared in Comic Relief story The Curse of Fatal Death and in the Doctor Who Unbound story Exile, though the former was a parody, and the latter included rules and mindsets which do not exist in the prime Doctor Who universe.

In Interference - Book One and Book Two, the Time Lord I.M. Foreman was portrayed as having changed sex as a result of regeneration, though the character is noted as having received the gift of regeneration when the process was still experimental and unstable. A cut line in The Unquiet Dead would have had Sneed remark to the Ninth Doctor, "I thought you'd be a woman," to which the Doctor would respond, "No, not yet." In Keeping up with the Joneses, the Tenth Doctor thought about the possibility of regenerating and acknowledged that it was "distantly possible" for him to regenerate into a female. He thought this would "keep life interesting." In The End of Time, the newly regenerated Doctor thinks for a short moment that "I'm a girl!" In The Doctor's Wife, the Doctor mentions the Corsair, who has regenerated into both male and female incarnations. Similarly, Harvest of Time revealed that one of 's potential future incarnations was female; a female Master going by the name later appeared in Deep Breath. The Night of the Doctor had the Sisterhood of Karn boast that they could control regeneration and give the Eighth Doctor the choice of "man or woman" for his next incarnation. Hell Bent showed the first on-screen male-to-female regeneration with the General's eleventh regeneration, shortly after The Black Hole had featured the first one ever. The General's first nine incarnations were female and she was pleased to return to a female incarnation. The Gallifrey series later depicted the first female-to-male regeneration through Trave's regeneration in Enemy Lines. The Twelfth Doctor's regeneration finally resulted in him becoming a woman, in 2017's Twice Upon a Time.

Skin colour
In The Sarah Jane Adventures story Death of the Doctor the Eleventh Doctor noted that his racial characteristics were not limited to white; he "can be anything." Although both actors to play K'anpo Rimpoche were Caucasian, Kevin Lindsay donned an accent and was made up to appear ethnically Tibetan. (TV: Planet of the Spiders) Romana was seen to regenerate into a blue-skinned form in TV: Destiny of the Daleks. Rassilon has been portrayed by white actors Richard Mathews, Timothy Dalton and Donald Sumpter on-screen while black actor Don Warrington was Rassilon's voice actor and cover-art model in Big Finish Doctor Who audio stories. In Let's Kill Hitler, River Song's second incarnation was black, while her first (in The Impossible Astronaut / Day of the Moon and A Good Man Goes to War) and third and last (appearing throughout series 4, 5, 6, 7, and 9) were white. Further, the General's eleventh regeneration was from a white man into a black woman. (TV: Hell Bent) One of Rindle's incarnations had darker skin than several previous incarnations. (AUDIO: The Lady in the Lake) Karlax regenerated into a body that had darker coloured skin than his previous body. (PROSE: Engines of War)

Number
Russell T Davies noted how firmly the concept of limiting Time Lords to thirteen lives, introduced in The Deadly Assassin, was lodged in fans' minds. Davies attempted to deliberately subvert the limit in Death of the Doctor, though he admits that fandom may resist his attempt to alter the programme's mythos. "When they came [to America] to launch The Eleventh Hour, I went along to this screening in LA and journalists put their hands up, and one of the first questions was, "What will happen when he reaches the thirteenth regeneration?" There's a fascinating academic study to be made out of how some facts stick and some don't – how Jon Pertwee's Doctor could say he was thousands of years old, and no-one listens to that, and yet someone once says he's only got thirteen lives, and it becomes lore. It's really interesting, I think. That's why I'm quite serious that that 507 thing won't stick, because the 13 is too deeply ingrained in the public consciousness. But how? How did that get there?"

- Russell T Davies

However, events depicted in Steven Moffat's The Time of the Doctor confirm the twelve-regeneration limit for Time Lords, and of the Doctor in particular, with the events of Time exploring the impact this has on the character having finally reached his limit, the Doctor facing his final death of old age until the Time Lords send him the energy for a new regeneration cycle. Several characters express a lack of knowledge over how many regenerations the Doctor currently has including the Doctor himself, (TV: Kill the Moon) Rassilon who had a hand in giving him the new regeneration cycle (TV: Hell Bent) and The Master. (TV: The Doctor Falls) Currently, a number has never been given for the number of regenerations the Doctor possesses in this second cycle. It is also unclear if the portion of regeneration energy that was stolen from the Twelfth Doctor by Davros impacted his ability to regenerate in any way. (TV: The Witch's Familiar)