Regeneration

Regeneration was the process by which Time Lords and others renewed themselves, causing a complete physical and often personality 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), 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, PROSE: Interference - Book Two) It could also be invoked by choice, whether voluntary (TV: Destiny of the Daleks, The Night of the Doctor, Nightmare in Silver) or involuntary. (TV: The War Games) Conversely, regeneration could be prevented by choice, although failure to regenerate from fatal damage was essentially a choice to die. (TV: Last of the Time Lords)

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. (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)

According to Madame Vastra, the Eleventh Doctor theorised that exposure over billions of years to the Untempered Schism contributed to the Time Lords' ability to regenerate. (TV: A Good Man Goes to War)

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's immune system was 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).

Appearance
During a regeneration, a Time Lord's body could shine with milky white light, (TV: Logopolis, 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 seemingly violent discharge of energy, (Orange/White) capable of causing damage known as regeneration energy. (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) 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) On the occasion that the Doctor had just been granted a brand new regenerative 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 his eighth incarnation onwards 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) with Rose Tyler just before the regeneration of the Ninth Doctor, (TV: The Parting of the Ways) with Jack Harkness, Donna Noble, and Rose when the Tenth Doctor regenerated during the 2009 Dalek invasion of Earth, (TV: The Stolen Earth) and when the Eleventh Doctor realised that Mels was regenerating. (TV: Let's Kill Hitler) Why he thought this was necessary became clear when the damage from 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 a thirteenth regeneration, it was the most destructive of all — so powerful that the town of Christmas was levelled, 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 their homes to take shelter. The Daleks that remained on the street were blown to pieces. (TV: The Time of the Doctor)

The Doctor is confirmed to have regenerated alone at the end of his seventh, war and tenth incarnations. His eighth incarnation also chose to regenerate by himself, but the Sisterhood of Karn were nearby when it took place. Though he demanded them to vacate his presence when he underwent his regeneration, Ohila approached him moments after he finished to see the result of his change, which she had engineered. (TV: The Night of the Doctor)

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 the aspects of their personality caused by "nurture", their basic personality, would not change, the "nature", their aspects, quirks, and habits, contribution to their personality would. (AUDIO: The Sirens of Time) With each incarnation, the Doctor's memory worked differently (PROSE: Nothing O'Clock), with some of his memories from before regeneration being lost even after the new incarnation had mentally stabilised unless they were specifically reminded of relevant events.

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 his first incarnation had only one heart. (PROSE: The Man in the Velvet Mask) According to the Eleventh Doctor, every regeneration was painful, (TV: Death of the Doctor) with the Seventh Doctor once describing regeneration as a good and bad feeling in the same way that driving a car very fast was a good and bad feeling, enjoying the exhilaration of the process but knowing that you were going to "die" at the end. (PROSE: The Room With No Doors)

More extreme changes were possible. Cavisadoratrelundar regenerated a complete body after being decapitated; 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). (PROSE: The Shadows of Avalon) The Eleventh Doctor at first thought he had become a woman before rediscovering his Adam's Apple, (TV: The End of Time) while the Ninth Doctor suggested two heads or none were possible. (TV: The Parting of the Ways) His eleventh incarnation stated clearly that he could become "anything". (TV: Death of the Doctor) The Corsair was a Time Lord/Time Lady whom the Doctor had known in both sexes. (TV: The Doctor's Wife) Time Lords could also change their skin colour, as mentioned by the Doctor, (TV: Death of the Doctor) and seen in Melody Pond's case when she regenerated from her second body into her third and final incarnation. (TV: Let's Kill Hitler)

In nearly all cases, Time Lord regenerations remained humanoid; one Time Lord, Lord Cardinal Zero, regenerated into an avian lifeform. (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 Gallifreyian 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).

After-effects
During the first few hours of the regeneration, the Time Lord might suffer from confusion, erratic behaviour, extended periods of unconsciousness, or memory loss. 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 extremely susceptible to side-effects from the regenerative process. His third incarnation was incapacitated for some time at Ashbridge Cottage Hospital. (TV: Spearhead from Space) Likewise, the fourth incarnation of the Doctor suffered acute delirium and memory loss and was placed under bed rest at UNIT Headquarters. (TV: Robot) It took some time for his newly-regenerated fifth incarnation to remember his own identity. (TV: Castrovalva) Following another regeneration, his sixth incarnation tried to strangle Peri to death before re-asserting control of himself, (TV: The Twin Dilemma) and in his following one, the Seventh Doctor was sufficiently weakened that the Rani was able to effectively brainwash him. (TV: Time and the Rani) His seventh regeneration was particularly traumatic for the Eighth Doctor, combining amnesia and emotional instability, though here the Doctor regenerated after he had been declared dead. (TV: Doctor Who) 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. (TV: The Christmas Invasion) After his twelfth regeneration, the Eleventh Doctor experienced powerful, rapidly alternating food cravings, declaring a certain type of food his favourite one minute, and saying he hated it the next. (TV: The Eleventh Hour) On two occasions the Doctor crashed the TARDIS following regeneration, first when the Tenth Doctor compulsively sped up the TARDIS and hit it against the wall of 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 storage shed in Leadworth. (TV: The End of Time, The Eleventh Hour)

There were exceptions. Save for some initial confusion over personal identity, the Second Doctor adjusted to his new body very quickly. (TV: The Power of the Daleks)

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. (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)

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) Even after the physical transformation, changes might occur. The Doctor was excessively tired after his 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 while recovering from his regeneration, (TV: Robot) while his eighth incarnation broke down a steel door bare-handed immediately following his regeneration. (TV: Doctor Who) Melody Pond, following her 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, the Doctor was forced to regress him mentally back to his third persona when he found himself facing a situation where the Third Doctor's skills would be better-suited to handle the crisis than his own abilities were. This happened during his sixth and seventh incarnations, the Sixth Doctor requiring 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 notable lacked his immediate two predecessors' skills at hand-to-hand combat, even if he remained a fairly capable swordsman. (TV: The King's Demons) Personal tastes could also vary, 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, or 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.

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)

Limitations
Though Time Lords could regenerate after severe injuries, regeneration was by no means guaranteed. Both the Tenth Doctor and River Song stated that a Time Lord could be killed permanently if killed at the right time during the regenerative process. (TV: The End of Time, The Impossible Astronaut) Maxil implied that a fatal blast from a staser (an energy weapon used by the Chancellory Guard on Gallifrey) could prevent regeneration. (TV: Arc of Infinity) Stabbing or shooting a Time Lord through both hearts at the same time, (PROSE: The Shadows of Avalon, World Game) or drowning, if it happened quickly enough (TV: Turn Left) could also end a Time Lord's life regardless of how many regenerations they had left. Acid could also destroy the ability to regenerate. (PROSE: Night of the Humans) 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 this environment. 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 to prevent regeneration. (PROSE: Cold Fusion)

Certain poisons could prevent the regeneration process, such as the poison from the Judas tree. (TV: Let's Kill Hitler) Retro-genitor radiation was specifically created by the Daleks to inhibit regeneration. (AUDIO: X and the Daleks) As well, the application of various medications, such as general anaesthetics from Earth, were known to disrupt the regenerative process. (TV: Doctor Who) The Fifth Doctor was not sure if he was going to regenerate after contracting Spectrox toxaemia. (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. (PROSE: The Eight Doctors) Due to the Time Lords relying on artron energy to power the regenerations, an artron inhibitor could prevent Time Lords from regenerating altogether, 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)

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 Gallifreyian 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 him "evolving" to become the vibrant new biosphere for an entire planet (PROSE: Interference - Book Two)

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 tranquility for a time afterward to allow the mind and body to properly readjust. (PROSE: Cold Fusion)

Regenerative cycle
Time Lords had a limited regenerative cycle of twelve regenerations, similar to the twelve hours on a clock, consisting of thirteen incarnations, after which they would suffer permanent death. (TV: The Deadly Assassin, Doctor Who, The Time of the Doctor) In one instance, the Tenth Doctor used up one of his cycle's regenerations while still keeping the same face. (TV: Journey's End, The Time of the Doctor) Time Lords could will themselves to die by regenerating when they had no more regenerations left to use, as Azmael chose to do. (TV: The Twin Dilemma) Rassilon apparently had physical reasons to impose this restriction. (AUDIO: Zagreus)

As with most such "rules", there were occasionally exceptions to the twelve regeneration limit. The High Council offered a new regenerative 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, during the Last Great Time War the Master was given the ability to regenerate at least once more. (TV: Utopia) It was implied he could regenerate further. (TV: Last of the Time Lords)

The Eleventh Doctor once claimed he could regenerate five hundred and seven times. (TV: Death of the Doctor) It was later revealed to not be true and that he only had twelve lives counting the Eighth Doctor's regeneration into the War Doctor and the Tenth Doctor using up a regeneration during his life but keeping the same face. 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) Time Lords could be revived with regeneration energy. Doing this, Melody Pond (in her third incarnation) burned up her remaining regenerations to save the Doctor's life and was hospitalised as a result. (TV: Let's Kill Hitler) The Doctor later used his regeneration 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)

Control over regeneration
Generally, the regeneration 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 began 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 his regenerations. (PROSE: Harvest of Time) showed this when he stated and achieved his intention to make his next regeneration as young as the Tenth Doctor appeared at the time. (TV: Utopia) Melody Pond announced she was "focusing on a dress size" in 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) In contrast to Romana, the Doctor did not seem to have much control over his post-regeneration appearance; after his fourth regeneration, he commented, "That's the problem with regeneration, you never quite know what you're going to get." (TV: Castrovalva) He restated this sentiment immediately prior to his tenth regeneration. (TV: The Parting of the Ways) He also nearly always examined himself or asked about his appearance. (TV: The Tenth Planet, Spearhead from Space, Robot, The Caves of Androzani, The Christmas Invasion, The End of Time) The Tenth Doctor referred to regeneration as "a lottery". (TV: The Day of the Doctor)

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 Doctors they would regenerate into later.

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 Landsknechte. (PROSE: Original Sin)

The Time Lords were apparently capable of controlling the regeneration of individual Time Lords, either forcing a regeneration, influencing the new appearance, (TV: The War Games) adding extra regenerations, (TV: The Five Doctors, Utopia, The Time of the Doctor) or removing later regenerations. (TV: The Ultimate Foe)

The Sisterhood of Karn also had elixirs that could control and trigger regeneration. These elixirs could determine traits to be held by the new incarnation such as appearance, sex, apparent age, and personality. When a Time Lord was dying and unable to regenerate normally, these elixirs could either hold back the Time Lord's death or initiate the regeneration process. (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 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) The Tenth Doctor was able to delay his regenerative process long enough to revisit each of his former companions of his own incarnation and his past selves as well before he finally regenerated. (TV: The End of Time, TV: Death of the Doctor)

Rassilon had discovered a form of true immortality beyond the regenerations known to the Time Lords, Rassilon kept this a secret believing it would be too dangerous to share. Borusa described Rassilon’s immortality as timeless perpetual bodily regeneration. (TV: The Five Doctors)

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 a day. (TV: The Christmas Invasion) When the Tenth Doctor underwent his own regeneration, the process was exceptionally violent and destructive to the TARDIS, likely a consequence of his delay of the procedure and/or the massive amounts of radiation he absorbed which caused the regeneration to start in the first place, though it hasn't been clear whether the severity of a regeneration-causing injury impacts the violence of the regeneration or not. (TV: The End of 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 his. (TV: Utopia) The Tenth Doctor also appeared to grimace in pain during the process. (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)

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 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)

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)

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 from the injury sustained from a Dalek Gunstick. This resulted in the appendage storing enough residual energy to actually grow an identical Time Lord when it came in contact with Donna Noble soon after. As a result, the Doctor used up a full regeneration (his eleventh of twelve overall), 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, The Time of the Doctor)

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)

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)

Aborted regeneration
Occasionally, a regeneration would fail and the regeneration 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. (PROSE: Timewyrm: Exodus)

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.

In fact, the Doctor seemed to regard his 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) However, they still regarded the other Doctors as them, to the extent that his seventh incarnation became ashamed of the actions of his sixth, 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) or the Third Doctor regarding his immediate predecessor as a distraction who would actually make him less effective. (TV: The Three Doctors) 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. (TV: The Day of the Doctor)

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)

The Doctor's attitude towards regeneration seemed to change during his later incarnations, considering it more like true death. In his 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) 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)

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 his incarnations, with Harriet calling 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)

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. (TV: The Hand of Fear)

The crew of the Minyans' ship, the P7E, could regenerate indefinitely, likely as a result of the interference by the Time Lords in their early history. Over time, they wearied of life. (TV: Underworld)

Mawdryn and his followers, who had stolen the Time Lords' regeneration technology, also had an apparently limitless 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. In other words, the TARDIS-generated energy that killed the Ninth Doctor was also apparently that which revived him as the Tenth.

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. Though his presence is never adequately explained, he is apparently corporeal enough to pilot the TARDIS and appear to characters other than the Doctor. He then combines with the dying Fourth Doctor to start the regeneration 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. Moreover, 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.

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 anesthesia, 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, TV: Death of the Doctor)

Aspects of both the ninth and tenth regenerations are invested in River Song's second regeneration, 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.

Some details about regeneration are given by stories that don't technically feature a regeneration. For instance, The Doctor's Wife establishes that Time Lords, or at the very least the Corsair, can change gender through regeneration.

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. Although not referred to by name, it was actually also seen previously in the Doctor Who TV movie.

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 short cut, 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 is completely identical. About a quarter of the 2011 episodes used that VFX in a way that wordlessly suggested regeneration.

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 has long been "fanon" that regeneration can cause a change of gender in Time Lords. Though the Doctor himself has not yet changed sex, a few stories still point out that it is a possibility.

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. Female versions of the Doctor appeared in Comic Relief parody story The Curse of Fatal Death and in the Doctor Who Unbound story Exile. 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. The Night of the Doctor has the Sisterhood of Karn boast that they can control regeneration, and give the Eighth Doctor a choice of "man or woman" for his next incarnation.

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) Rassilon has been portrayed by white actors Richard Mathews and Timothy Dalton 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, one of River Song's incarnations is black, while her first (in The Impossible Astronaut / Day of the Moon and A Good Man Goes to War) and last (appearing throughout series 4, 5, 6 and 7) are white.

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 The Doctor in particular; the impact this has on the character having finally reached his limit. (TV: The Time of the Doctor).