Regeneration


 * This article is about the Time Lord physiological process. For other uses of Regeneration, see Regeneration (disambiguation)

Regeneration was the process by which Time Lords and near-Time Lords renewed themselves, causing a complete physical (and often, emotional) change. It could happen at will or because of severe illness or injury.

Scientific explanation
Different explanations were given for the process behind regeneration.


 * One explanation was 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 and personality of the former incarnation. Rassilon intended this mechanism only for the Gallifreyan elite. He also inputted a parameter of twelve regenerative cycles to avoid decaying biogenic molecules. (BFA: Zagreus)
 * Another theory attributed regeneration to a "nanomolecular virus" that rebuilt the body much like the "self-replicating biogenic molecules". (REF: The Gallifrey Chronicles)
 * A third theory was that Time Lords had triple-helix DNA: the third strand was added by Rassilon to make regeneration possible. (MA: The Crystal Bucephalus)
 * One partial explanation of the process linked it to the release of massive amounts of a hormone called lindos in moments of extreme trauma; the hormone triggered the regeneration. Newly regenerated Time Lords could be identified by elevated levels of lindos in their system. (DWN: The Twin Dilemma, BFA: Unregenerate!)
 * At one point it was stated that Time Lords had "packets" of regeneration energy within their bodies, one for each life. These packets could be physically removed from a Time Lord's body, essentially robbing them of their regenerations. (DW: Mawdryn Undead)
 * Most recently it was stated that exposure over billions of years to the Untempered Schism caused the Time Lords to be able to regenerate. (DW: A Good Man Goes to War )

Appearance
During a regeneration, a Time Lord's body shone with milky white light (DW: Logopolis, Time and the Rani, Doctor Who) or with a violent-seeming discharge of bio-energy. (DW: The Parting of the Ways, Utopia, The Stolen Earth, The End of Time, The Impossible Astronaut). In other cases, there was no visible energy discharge. (DW: Planet of the Spiders, Destiny of the Daleks)

Just before his ninth regeneration, the Doctor warned his companion to keep away from him. (DW: The Parting of the Ways, The Stolen Earth) The damage to the TARDIS from the Tenth Doctor's regeneration into the Eleventh, enough to force a complete reconstruction, may have been an example of the dangers the regeneration energy could pose. (DW: The End of Time) However, other regenerations occurred with other individuals in close proximity. (DW: The Tenth Planet, Planet of the Spiders, Logopolis, The Caves of Androzani, Time and the Rani)

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" would not change, the "nature" contribution to their personality would. (BFA: The Sirens of Time) One dubious source stated that Time Lords of the Oldblood Houses, born with just one heart, grew a second heart on regeneration. This included the Doctor, who in his first incarnation had only one heart. (MA: The Man in the Velvet Mask) According to the Doctor, every regeneration was painful. (SJA: Death of the Doctor)

More extreme changes were possible. Cavisadoratrelundar regenerated a complete body after being decapitated; the process was cut short when he was stabbed through both hearts. (EDA: The Shadows of Avalon) The Eleventh Doctor feared he had become a woman, (DW: The End of Time) while the Ninth Doctor suggested two heads or none were possible (DW: The Parting of the Ways). His eleventh incarnation stated clearly that he could become "anything". (SJA: Death of the Doctor) The Corsair was a Time Lord/Time Lady whom the Doctor had known in both sexes. (DW: The Doctor's Wife) Time Lords could also change their skin colour. (DW: Let's Kill Hitler)

In nearly all cases, Time Lord regenerations remained humanoid; one Time Lord, Lord Cardinal Zero, regenerated into an avian lifeform. (BFA: Spring) Though a healthy body seems 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. (DW: The End of Time)


 * The Doctor's ability to regrow a severed limb soon after regeneration (DW: The Christmas Invasion) suggests the possibility that had the eleventh incarnation, for example, discovered a deficiency - a missing leg, for example - he might have been able to rectify the situation.

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), and a Time Lord could suffer random spasms as the regeneration settled (DW: 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 (DW: Spearhead from Space). Likewise, the fourth incarnation of the Doctor suffered acute delirium and memory loss, and was placed under bed rest at U.N.I.T. Headquarters(DW: Robot). It took some time for his newly-regenerated fifth incarnation to remember his own identity. (DW: Castrovalva) Following another regeneration, he tried to strangle Peri to death before re-asserting control of himself (DW: The Twin Dilemma), and in his following one, was incapacitated the The Rani was able to effectively brainwash him (DW: Time and the Rani). His seventh regeneration was particularly traumatic, combining amnesia and emotional instability (DW: Doctor Who. On two occasions he wound up losing control of and crashing the TARDIS following regeneration, first against the wall of the Powell Estate, then into the storage shed of the Pond residence (DW: Children in Need Special, The Christmas Invasion, The Eleventh Hour). After his tenth regeneration, the Doctor experienced powerful, rapidly alternating food cravings, declaring a certain type of food his favourite one minute, and saying he hated it the next. (DW: The Eleventh Hour)

A Zero Room could help with the process, as it removed all outside distractions. (DW: Castrovalva) After his first regeneration, the Second Doctor implied that the TARDIS itself helped the process along. (DW: The Power of the Daleks) For other species, a metamorphic symbiosis regenerator might offer help when regenerations failed, though whether this true of Time Lords is unknown. (DW: Mawdryn Undead)

Were a Time Lord knocked unconscious, the whole process might start all over again (DWN: The Power of the Daleks), but this was not a certainty (DW: The Eleventh Hour). A Time Lord fatally injured a second time during the regeneration would die from the initial injury, unable to complete the regeneration. (DW: The Impossible Astronaut) After a while, the Time Lord's body would settle down, though they could regrow limbs within fifteen hours of the regeneration due to residual energy. (DW: 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. (DW: Robot) The Fifth Doctor's hair went from longer to shorter to longer in the space of a few days. (MA: 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 (DW: Robot), his eighth incarnation broke down a steel door bare-handed immediately following his regeneration. (DW: 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. (DW: Let's Kill Hitler)

However, some Time Lords were able to regenerate with little or no overt complications, as in the case of Romana. (DW: Destiny of the Daleks)

Limitations
Though Time Lords could regenerate after severe injuries, regeneration was by no means guaranteed. Maxil implied that a fatal blast from a staser (an energy weapon used by the Chancellory Guard on Gallifrey) could prevent regeneration. (DW: Arc of Infinity) Stabbing or shooting a Time Lord through both hearts at the same time (EDA: The Shadows of Avalon, PDA: World Game), burning out both hearts (DW: Silence in the Library / Forest of the Dead) or drowning, if it happened quickly enough (DW: 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. (NSA: Night of the Humans). Some illnesses, such as Chen-7, could preclude regeneration. (DW:The Girl Who Waited) If the regeneration were interrupted with a fatal injury, the regeneration would fail. (DW: The Impossible Astronaut) Certain poisons could prevent the regeneration process, such as the poisoned lipstick River Song possessed. (DW: Let's Kill Hitler). As well, the application of various medications, such as general anaesthetics from Earth, were known to disrupt or destroy the regenerative process (DW: Doctor Who).

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. (MA: Cold Fusion)

Regenerative cycle
Time Lords had a limited regenerative cycle of twelve regenerations, consisting of thirteen incarnations, after which they would suffer permanent death. (DW: The Deadly Assassin, Doctor Who) Time Lords could will themselves to die by regenerating when they had no more regenerations left to use, as Azmael chose to do. (DW: The Twin Dilemma) Rassilon apparently had physical reasons to impose this restriction. (BFA: Zagreus)

As with most such "rules" there were occasionally exceptions to the twelve regeneration limit. The High Council, offered the Master a new regenerative cycle if he rescued the various incarnation of the Doctor from the Death Zone. (DW: 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. (DW: Utopia)

In his Eleventh incarnation, the Doctor claimed he could regenerate five hundred seven times. (SJA: Death of the Doctor) However whether the Doctor was speaking the truth is unknown.

Time Lords could be revived with regeneration energy. Doing this, Melody Pond (in her third incarnation) used up her remaining regenerations to save the Doctor's life and was hospitalised as a result. It is unknown if this act gave the Doctor her remaining regenerations. (DW: Let's Kill Hitler)

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 seemed to regenerate on a whim (DW: Destiny of the Daleks), while Azmael began a thirteenth regeneration to end his life. (DW: The Twin Dilemma)

The degree of control that Time Lords had over their end appearance was unclear. The Master made his next regeneration as young as the Tenth Doctor appeared at the time, though since most regenerations led to younger bodies this may have been a coincidence (DW: Utopia). Melody Pond announced she was "concentrating on a dress size" in her final regeneration. (DW: Let's Kill Hitler) Romana seemed adept enough at the process to custom design her new form, trying several bodies before finally deciding on a copy of Princess Astra. The Doctor criticised Romana for taking on the form of another person, suggesting such things were not unheard of. (DW: 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 (DW: Castrovalva)." He would restate this sentiment immediately prior to his ninth regeneration (DW: The Parting of the Ways)
 * However as the Doctor's regenerations have thus far either been forced on him or been triggered when an incarnation has been mortally wounded, it may be that the appearance of the next incarnation can be controlled if the regeneration is voluntarily triggered (as was Romana's).


 * Melody Pond commented after her regeneration, that she could make herself gradually younger over her life. Whether this was part of the regenerative process or not is unknown.

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. (DWN: The Twin Dilemma)

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. (NA: Original Sin)

The Time Lords were apparently capable of controlling the regeneration of individual Time Lords, either forcing a regeneration, influencing the new appearance (DW: The War Games) or removing later regenerations. (DW: The Ultimate Foe)

With effort, Time Lords could resist regeneration, effectively committing suicide. (DWN: The Power of the Daleks) The Master did so after being shot, ostensibly to avoid becoming the Tenth Doctor's eternal prisoner. (DW: 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. (BFA: 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. (DW: 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. (DW: The End of Time, SJA: Death of the Doctor)

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. (DW: 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. (DW: 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 slipped into a coma-like state for most of a day (DW: Children in Need Special, 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 (DW: ''The End of Time). ''

Regeneration, especially later ones, could be painful. Melody Pond screamed during her second regeneration (DW: Let's Kill Hitler), as did the Master during his transition into his "Harold Saxon" incarnation. (DW: Utopia). Both Ninth and Tenth Doctors also appeared to grimace in pain during the process (DW: The Parting of the Ways, The End of Time).

Meta-crisis
A Time Lord could prevent death and regeneration by focusing the regenerative energies into a severed appendage, like the Doctor's hand. His hand siphoned off the excess energy that would have changed his appearance while the Tenth Doctor used just enough to heal himself from the injury sustained by a Dalek Gunstick. This resulted in the appendage storing enough energy to actually grow an identical Time Lord when it came in contact with Donna Noble. As a consequence, the Doctor appeared to regenerate and heal but did not change. (DW: Journey's End)


 * It is unknown if this used up one of the Doctor's regenerations.

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. (EDA: Interference - Book One, Interference - Book Two) Romana, prior to her regeneration into her second incarnation, appeared to have taken on a near-human blue-skinned form. (DW: Destiny of the Daleks) However, one account held that the TARDIS itself, rather than Romana, adopted this shape. (ST: 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 could 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 (NA: 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". (DW: Doctor Who: The Movie) In fact, the Doctor seemed to regard his previous incarnations as different individuals, capable of interacting and working with each other (DW: The Three Doctors, The Five Doctors, The Two Doctors, Time Crash).Time Lords such as the War Chief were unconcerned about wasting regenerations, while others such as the Doctor warned not to waste them. (MA: 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. (EDA: The Scarlet Empress)

The Doctor's opinions 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. (DW: 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." (DW: The Waters of Mars, The End of Time)

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 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. (DW: 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. (DW: Mawdryn Undead)
 * Chris Cwej was regenerated by force to survive radiation poisoning. (BNA: Tears of the Oracle)
 * K9 Mark I regenerated by use of a Regeneration unit after self-destructing to defeat a group of Jixen Warriors. (K9TV: Regeneration) He later regenerated again after exhausting his power core to defeat the Trojan. (K9TV: The Eclipse of the Korven)
 * The Cinder was a sentient spark which grew into a Phoenix made of living flame. When extinguished, it slowly regenerated from a single surviving spark back into its Phoenix form. (CC: Frostfire)
 * Melody Pond, a partial Time Lord, after being conceived in The Doctor's TARDIS, regenerated on two occasions. Her first regeneration happened after near-death from an unspecified illness. (DW: Day of the Moon) Her second regeneration happened when she was accidentally shot by Nazi leader, Adolf Hitler. (DW: Let's Kill Hitler)

The Doctor

 * First Regeneration/Second Incarnation: Grew increasingly ill as Mondas drained his energy as well as Earth's, coupled with his body's advanced age.(DW: The Tenth Planet)
 * Second Regeneration/Third Incarnation: Enforced upon him by the Time Lords as part of his punishment for breaking the Laws of Time. (DW: The War Games)
 * Third Regeneration/Fourth incarnation: Caused by exposure to The Great One's highly unstable web of Metebelis crystals. (DW: Planet of the Spiders)
 * Alternate Third Regeneration/Fourth incarnation: Shot by Magdalena on the planet Dust after defeating the Remote. (EDA: Interference - Book Two) This timeline was created when the Eighth Doctor accidentally diverted his third self's TARDIS, and was later undone. (EDA: The Ancestor Cell).
 * Fourth Regeneration/Fifth incarnation: Unintentionaly caused by the Fourteenth Master by rotating the Pharos Project radio telescope to the point where he had a fatal fall to the ground. (DW: Logopolis)
 * Fifth Regeneration/Sixth Incarnation: Caused by contact with unrefined spectrox, causing a prolonged death, which he felt he would not be able to regenerate from. (DW: The Caves of Androzani)
 * Sixth Regeneration/Seventh incarnation: Caused by injury from the Rani's attack on the Doctor's TARDIS (DW: Time and the Rani). Accounts indicate the Doctor hit his head (NA: Head Games); however, he was also suffering from a chronal energy drain after his confrontation with the Lamprey (PDA: Spiral Scratch).
 * Seventh Regeneration/Eighth incarnation: Caused by gunshot wounds from the crossfire of an Earth gang war and exacerbated by hospital surgery which attempted to explore his unearthly physiology, but only succeeding in damaging his cardiovascular system far beyond repair. (DW: Doctor Who)
 * Eighth Regeneration/Ninth Incarnation: Unknown, but believed to have been caused by injuries sustained in the Last Great Time War. (IDW: The Forgotten)
 * Ninth Regeneration/Tenth incarnation: Caused by cellular degeneration from absorbing the energies of the time vortex from Rose Tyler to save her life and restore the TARDIS. (DW: The Parting of the Ways)
 * During tenth incarnation: Partial regeneration after being hit by Dalek fire; (DW: The Stolen Earth) aborted by channelling the energy into his severed hand, leading to the two-way biological metacrisis that would spawn the Meta-Crisis Tenth Doctor and the Doctor Donna (DW: Journey's End)
 * Tenth Regeneration/Eleventh Incarnation: Caused by severe radiation poisoning when the Doctor sacrificed himself to spare Wilfred Mott, absorbing all the radiation from the nuclear bolt used to power the Immortality Gate. It took a long time later rather instantly. (DW: The End of Time)

Alternate Doctors

 * The Doctor: Shot by Ruth after she learned the truth about his role in her father's death; she shot him again when he regenerated and so on. (BFDWU: Full Fathom Five)

Romana

 * First to Second incarnation: Decided it was time for a change, regenerating into several different forms before settling on the features of Princess Astra. (DW: Destiny of the Daleks)
 * Second to Third incarnation: Regenerated in preparation for the Second War in Heaven, developing a body more suited for combat. (EDA: The Shadows of Avalon)
 * Third to Fourth incarnation: Managed to survive the Second War in Heaven, although this may have entailed a regeneration (PDA: Tomb of Valdemar)

The Master

 * When his TARDIS was pulled into a black hole by the Second Doctor, Koschei used up most his regenerations escaping it. (MA: The Dark Path)
 * During the Last Great Time War, the Time Lords resurrected the Master, apparently giving him a new regeneration cycle in the process (DW: The Sound of Drums) as, when the Master was shot by Chantho, he regenerated into a new incarnation. (DW: Utopia)

Rassilon
Rassilon was said to have achieved a cycle of perpetual regeneration, becoming immortal. (DW: The Five Doctors)

K'anpo Rimpoche
Attacked by Eight Legs. (DW: Planet of the Spiders)

Borusa
Borusa regenerated at least three times, and had at least four lives. (DW: The Deadly Assassin, DW: The Invasion of Time, DW: Arc of Infinity, DW: The Five Doctors)

Rallon
Initiated all twelve of his regenerations to stop the Celestial Toymaker. (PDA: Divided Loyalties)

Azmael
Azmael used all his twelve regenerations in unknown circumstances. In his Thirteenth and final body he regenerated past his limit, killing him and Mestor, who possessed Azmael's body after his own was destroyed. (DW: The Twin Dilemma)

Iris Wildthyme
Regenerated following a lengthy period of illness following the consumption of a live Kaled mutant. (EDA: The Scarlet Empress)

The War Chief
Underwent a faulty regeneration after being shot by War Lords. This resulted in his new form appearing like two bodies fused together. (NA: Timewyrm: Exodus)

Mortimus
Mortimus regenerated at least once. (NA: No Future, BFA: The Book of Kells)

I.M. Foreman
As a priest, he had been given the gift of regenerations. This made twelve different individuals, who were created by absorbing the DNA around him. (EDA: Interference - Book One, Interference - Book Two)

The Corsair
The Corsair regenerated several times, with "a couple" of his incarnations being female. He was back to male by the time he was killed by House. (DW: The Doctor's Wife)

River Song

 * First to second incarnation: As a little girl, Melody Pond, later known as River Song, regenerated on the streets of New York City in 1970 due to a terminal illness. (DW: Day of the Moon)
 * Second to third incarnation: Melody later regenerated in Hitler's private study in 1938 after the Führer accidentally shot her while fearing the Teselecta. She later used up all of her remaining regenerations to heal the Doctor. (DW: Let's Kill Hitler)

History
Regeneration was introduced to the mythos of Doctor Who in order to solve a practical staffing problem: producer Innes Lloyd needed to find a way to exit William Hartnell in a way that allowed the show to keep running.

It is uncertain who, precisely, came up with the idea of regeneration, but 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)

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", reprinted in The Second Doctor Handbook p.24

Initially, the concept wasn't called "regeneration" at all, but rather "rejuvenation". 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.

Tradition has it that Hartnell himself thought up the idea of the Doctor being able to change into a new body as a means of keeping the series going after his departure. In The Power of the Daleks, the next story, the Doctor said that he had been "renewed", and also said that the change was partly due to the TARDIS, partly to himself, without elaborating. Originally, the production team meant for the Doctor to have used the TARDIS to reverse time and "rejuvenate" himself, though they may have abandoned this explanation by the time Patrick Troughton took over the role.
 * The change in the Doctor's appearance was meant to occur several stories earlier, during The Celestial Toymaker, with the Toymaker capriciously having changed the Doctor's appearance out of spite. (The Doctor is invisible and unable to speak for most of the story anyway.) The reason for the change of plan is unknown. Either it was decided to retain Hartnell in the role for a few more stories, or the actor was accidentally issued with a new contract by mistake, making it impossible to recast the Doctor at that moment. (A similar plot device would occur in The Mind Robber to cover Frazer Hines' temporary replacement by his cousin, Hamish Wilson, after Hines contracted an illness.)
 * The changeover was not actually referred to as "regeneration" until the end of Planet of the Spiders, when the Third Doctor (Jon Pertwee) changed into the Fourth (Tom Baker). Prior to this, the Doctor was simply described as having "changed his appearance."
 * Despite being the Doctor's "greatest enemy", the Daleks have directly or indirectly caused fewer of the Doctor's regenerations than his own race (the Time Lords). The Daleks have only been in two adventures that forced the Doctor to regenerate (the Eighth to Ninth Doctor regeneration's circumstances is unknown), whereas a Time Lord has had a hand in three of his regenerations.
 * In the original 1963-89 series, plus the 1996 TV movie, each regeneration was treated differently on screen, using a variety of special effects ranging from simple cross-fades to a CGI "morph" in the 1996 film. With the return of Doctor Who in 2005, producer Russell T Davies decided regenerations would take a consistent form, with the Time Lord in question standing upright, arms outstretched, and blinding energy flying out from the head, arms, and legs. So far, the seven regenerations depicted on screen (four featuring the Doctor in The Parting of the Ways, The Stolen Earth, The End of Time and The Impossible Astronaut, one featuring the Master in Utopia, and two featuring River Song in Day of the Moon and Let's Kill Hitler) have taken on this form.
 * Regeneration has been played for laughs in non-canonical spoofs such as The Curse of Fatal Death, in which the Doctor's repeated accidental deaths enabled five actors to play the role within minutes of one another.
 * Regeneration is used in the video game The Mazes of Time. Should the Eleventh Doctor or Amy Pond (who is not a Time Lord), be killed, they will regenerate.

Alternate possibilities
"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? It’s fascinating, it's really weird. Anyway, that'll be my book in my retirement!"
 * Fans and the general public have long speculated as to whether the Doctor could change sex as a result of a regeneration. 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 the non-canonical The Curse of Fatal Death and in the Doctor Who Unbound story Exile. In the audio drama The Two Irises, Iris Wildthyme encounters a future, male incarnation of herself. In The End of Time the newly regenerated Doctor shouts out "I'm a Girl!", however this was just seconds after regeneration and it is possible this was post-regenerative trauma. In The Doctor's Wife the Doctor mentioned his friend, the Corsair, who had regenerated as both male and female.
 * In Death of the Doctor the Doctor noted that his racial characteristics were not limited to white; he "can be anything." This is the first reference to such a possibility on-screen, but not in canon; 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, it turned out that Melody Pond's second incarnation was black, while her third was white.
 * 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 deliberately broke the limit in Death of the Doctor, though he admits that fandom may resist his attempt to alter the programme's mythos so.

- Russell T Davies