Regeneration


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

Regeneration was the process by which Time Lords, and a few other life-forms, could renew themselves by complete physical change, either at will or because of near-fatal illness or injury.

Appearance
During a regeneration, the body often shines with milky white light also known as bio-energy (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) In other cases, there was no visible energy discharge. (DW: Planet of the Spiders, Destiny of the Daleks)
 * The colour of bio-energy released during regeneration may be different between individual Time Lords, as the Doctor's have released golden-orange energy while the Master's released brilliant white energy.

Just prior to his ninth regeneration, the Doctor warned his companion to keep away from him (during a partial regeneration, Jack did this as well). This would seem to imply that the energy released during the process posed some level of danger to humans, and possibly that them being too close may pose a danger to the Time Lord. (DW: The Parting of the Ways, The Stolen Earth) The damage done to the TARDIS during the Tenth Doctor's regeneration into the Eleventh (enough to force a complete reconstruction) may be an example of the potential dangers the regeneration energy can pose to those who stand too close. (DW: The End of Time) However at least one regeneration occurred with other individuals seen in close proximity (although this was assisted by the transient Watcher, who may have helped to contain the regenerative energy). (DW: Logopolis)

Physical and mental change
In the case of the Doctor, the new form had often been physically younger and healthier (in human terms) than the Time Lord's previous incarnation, though this wasn't always the case, see, for example, the contrast between his fifth and sixth incarnations. Other Time Lords have regenerated into both younger (Romana) and older (Borusa) bodies. The Time Lord might have gained or lost height and body mass. After each regeneration there was often a marked change in personality. During the process of regeneration there were the genetic equivalent of 'bit errors' in the DNA of the regenerated cells. This was what caused the appearance of the Time Lord to change, because even the cells of the brain regenerated as well, and their brain chemistry and organisation would change. Although the aspects of their personality caused by "nurture" would not change, the "nature" contribution to their personality would change. (BFA: The Sirens of Time) Time Lords of the Oldblood Houses, born with just one heart, grew a second heart upon regeneration. This included the Doctor, who in his first incarnation had only one heart. (MA: The Man in the Velvet Mask)
 * When one of the Doctor's hearts stopped he wondered aloud how humans could cope with just one, suggesting he had never experienced a single heartbeat. (DW: The Shakespeare Code) However as he was in his tenth incarnation at the time, it may have been so long since he last had the experience he may have forgotten what it was like.

The Eleventh Doctor feared that he had become a woman, suggesting that it was a possibility when it came to regeneration. (DW: The End of Time) This was justified by the fact that regeneration was very random and, as the Ninth Doctor told Rose Tyler, "You never know what you're going to end up with". He also stated prior to that comment that he could even have grown another head due to regeneration, though he may have been joking as he also suggested that an unlikely "no head" outcome was possible. (DW: The Parting of the Ways) However, later comments by his eleventh incarnation state clearly that he could become "anything". (SJA: Death of the Doctor) Otherwise, all known naturally occurring Time Lord regenerations have remained humanoid with all vital limbs and organs in place. Nonetheless, 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) According to the Doctor, every regeneration was painful. (SJA: Death of the Doctor)
 * 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 will often suffer from confusion, erratic behaviour and memory loss. The Doctor, in particular, though not so much other Time Lords, has exhibited mood swings and confusion. It took some time for his newly-regenerated fifth incarnation to remember his own identity. (DW: Castrovalva) On one occasion, he attempted to strangle Peri to death before re-asserting control of himself (DW: The Twin Dilemma) and on another he almost crashed the TARDIS (DW: Children in Need Special) Following 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 this, as it removed all outside distractions. (DW: Castrovalva)

After a regeneration, the Time Lord may have difficulty "steering" their new body, and may also suffer random spasms as the regeneration settles. Several hours after the Time Lord's appearance has changed, the regeneration may still be going on in some way. (DW: The Eleventh Hour)

If they are knocked unconscious, the whole process might be started all over again (DWN: The Power of the Daleks), but this is not a certainty (DW: The Eleventh Hour). After a while, the Time Lord's body will have settled down, though they can regrow limbs within the first 15 hours of the regeneration due to having enough residual energy. (DW: The Christmas Invasion) After his first regeneration, the Second Doctor implied that his TARDIS helped the process along. (DW: The Power of the Daleks) Some Time Lords, however, may regenerate with little or no overt complications, for example, Romana. (DW: Destiny of the Daleks) Even after the physical transformation, changes might occur, as the Fourth Doctor believed could happen. The Doctor also appeared to be excessively tired after his third regeneration, deliberately falling asleep in many odd locations. (DW: Robot) The Fifth Doctor's hair, for instance, 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 displays greater strength then usual; The Doctor's fourth incarnation was able to karate-chop a brick in half while recovering from his regeneration (DW: Robot), and his eighth incarnation managed to break down a steel door bare-handed immediately following his. (DW: Doctor Who) This strength may be caused in a similar process as human adrenaline spikes.

Limitations
Though Time Lords could regenerate after severe injuries — one Time Lord even beginning to regenerate a complete body after being decapitated (EDA: The Shadows of Avalon), the process only being cut short when a Time Lord is stabbed through both hearts before it can finish — regeneration does not seem to be guaranteed. The Ninth Doctor, for example, was at one point convinced that he was going to die at the hands of the Gelth (DW: The Unquiet Dead), although it is possible he meant the particular incarnation, and there have been numerous occasions in his lifetimes where survival, including regeneration, was not assured. 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), or drowning, if it happened quickly enough (DW: Turn Left) could also end a Time Lord's life regardless of how many regenerations they have left. Burning out the hearts simultaneously would have the same effect as stabbing or shooting them simultaneously (DW: Silence in the Library/Forest of the Dead). Acid could also render the ability to regenerate impossible (NSA: Night of the Humans)

As well, the interference of some medications, such as anaesthesia, may disrupt or destroy the regenerative process. (DW: Doctor Who)

Regenerative cycle
The Time Lords had a limited regenerative cycle of twelve regenerations, consisting of thirteen incarnations, after which they had no more regenerations and would suffer permanent death. (DW: The Deadly Assassin, Doctor Who) Time Lords can will themselves to die by regenerating when they have no more regenerations left to use, as Azmael chose to do (DW: The Twin Dilemma) and can also, with enough effort, prevent a regeneration and die permanently should they desire. (DW: Last of the Time Lords) Other life forms (for example, the Minyans) could have hundreds of regenerations. 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 complied with rescuing 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 once more. (DW: Utopia)

The crew of the Minyans' ship the P7E, however, had had about a thousand regenerations, by which time, however, 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 of a kind of de facto punishment by the Time Lords for stealing their technology. (DW: Mawdryn Undead)

In his Eleventh incarnation, the Doctor claimed he could regenerate 507 times. (SJA: Death of the Doctor)

Attitude toward regeneration
As noted above, regeneration is not guaranteed, as the Doctor on numerous occasions believed he was in danger of actually dying. Even with regeneration a possibility, the Doctor has come to consider such a change as being nonetheless a "death". In recollecting the events surrounding the Master's attempt at stealing the Eye of Harmony, the Eighth Doctor referred to his incarnations as "lives". (DW: Doctor Who: The Movie) 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, saying that regeneration was treated on Gallifrey the same way sex is on Earth. (EDA: The Scarlet Empress)

The Doctor's opinions towards regeneration seemed to change in his later regenerations, considering it more 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 regarding 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)

Control over regeneration
Generally, regeneration is initiated when a Time Lord has been too badly injured to survive. However in some cases Time Lords have been known to exercise control over the process. Romana appeared to regenerate on a whim, (DW: Destiny of the Daleks) while Azmael initiated a thirteenth regeneration in order to end his life (DW: The Twin Dilemma).

The degree of control that Time Lords have over their end appearance is unclear. The Master was able to make his next regeneration as young as the Tenth Doctor appeared at the time, though since most regenerations lead to younger bodies this may have been a coincidence. (DW: Utopia) Romana seemed adept enough at the process to custom design her new appearances, 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 appear 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), and would restate this 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 (such as Romana's).

While skilled Time Lords can choose their new form with a voluntarily induced regeneration, the process has the potential to go horribly wrong and leave the Time Lord in a severely misshapen body. The problem is then often exacerbated by the Time Lord trying to solve the problem by immediately starting another regeneration instead of obtaining medical assistance, which only amplifies the defects in the regeneration. The end result of several too close regenerations, whether self-initiated or not, is inevitably a mutated monstrosity that can 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 could give the Time Lord a new set of genes, allowing them to fool genetic sensors. The Seventh Doctor used this method to on the planet Purgatory in order 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 some difficulty, Time Lords could resist regeneration, thereby 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) Later events showed, however, that he had a complex plan already in motion to provide for the resurrection of that incarnation, rather than using up a regeneration. (DW: The End of Time) Similarly, the Fifth Doctor once threatened System with resisting regeneration in order to stop the device from learning the biological details of the act. (BFA: The Gathering) This is not always an option, however, as the Doctor notes fearfully that while his companion can die only once, he may have to repeatedly regenerate and live out all of his life times when the TARDIS stalls in the middle of space. (DW: Vengeance on Varos)

The Tenth Doctor appeared to resist the regenerative process long enough to revisit each of his former companions (both those of his own incarnation and his past selves as well). This resistance to the process may have resulted in the particularly destructive nature of his regeneration, which partially destroyed the TARDIS control room.

Time Lords seemingly developed a means for artificial life forms to undergo regeneration as well; the robot dog K9 Mark I was fitted with a Regeneration unit which allowed him to upgrade to a significantly more advanced model following his self-destruction. (K9TV: Regeneration)

''It is possible that the Korven and not the Time Lords created the regeneration unit due to K9's affiliation with them. (K9TV: Hound of the Korven)''

Regenerative difficulty
While most regenerations seemed to cause moments of mental instability (with some degree of temporary amnesia often noted), some offered particularly profound instances of physical peril. The Fifth Doctor feared that his fourth regeneration "was failing" when he found himself reverting into previous personas, and could only be righted with the use of the Zero Room of the TARDIS. (DW: Castrovalva) Later, the Eighth Doctor claimed that anesthesia had "nearly destroyed the regenerative process" during his seventh regeneration, as a possible explanation for the particularly severe amnesia he suffered afterwards. (DW: Doctor Who) Still later, one of the Tenth Doctor's hearts stopped beating for a prolonged period of time, and he began to exhale regenerative energy when his ninth regeneration went "a bit wrong", after which he slipped into a coma-like state for most of a day. (DW: Children in Need Special, The Christmas Invasion)

For other species, a metamorphic symbiosis regenerator was said to be able to provide help when regenerations failed. (DW: Mawdryn Undead)

Meta-crisis
A Time Lord can 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 a nearly identical Time Lord after 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
Though they did not undergo normal regenerations, as noted elsewhere, Mawdryn underwent many mutations of physical form. (DW: Mawdryn Undead) 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 in to her second incarnation, appeared to have taken on a near-human blue-skinned form. (DW: Destiny of the Daleks)


 * One account states that the TARDIS itself, rather than Romana, adopted this shape. (ST: The Lying Old Witch in the Wardrobe)

Aborted regeneration
Occasionally, a regeneration will fail and the regeneration will abort. Though the Time Lord will have regenerated, they are severely deformed. Though Time Lord technology can treat this, in some occasions the damage will be too severe to fix.

After being shot by the War Lords, the War Chief was barely able to survive. While being taken back to the War Lords' planet, his body attempted to regenerate. Due to the massive injuries and the lack of medical care, this regeneration aborted, which meant he had two conjoined individual bodies, poorly fused together. (NA: Timewyrm: Exodus)

Many times after a regeneration has occured, a Time Lord may sometimes feel pain. This may be due to either the regeneration process being incomplete or failing. The Fourth Doctor didn't feel any pain at all, noting that he had no trouble with his regeneration. (DW: Robot) Sometimes after a regeneration Time Lords suffered amnesia, and have been known to attack people, as when the Sixth Doctor attacked Peri Brown after regeneration. (DW: The Twin Dilemma)

Scientific explanation
The exact mechanism that makes regeneration possible has not been stated. Varying explanations may or may not be compatible with each other.


 * One explanation was that Cardinal Rassilon had been investigating a method of regenerating decaying and diseased tissue via a series of permanently carried self-replicating biogenic molecules. The cells of a Gallifreyan body could be repaired, restored and re-organised. This would result in a wholly new physical form. The brain cells would similarly be rearranged, though to a lesser degree, thus ensuring the new incarnation will replicate the memories and personality of the former incarnation. Rassilon intended for this mechanism only to be used upon the Gallifreyan elite. He also inputted a parameter of 12 regenerative cycles to avoid decaying biogenic molecules. (BFA: Zagreus)


 * Another theory attributes regeneration to a "nanomolecular virus" that rebuilds the body much like the "self-replicating biogenic molecules". (REF: The Gallifrey Chronicles)


 * A third theory is that Time Lords have triple-helix DNA: the third strand was added by Rassilon to make regeneration possible. (MA: The Crystal Bucephalus)


 * One partial explanation of the process links it to the release of massive amounts of a hormone known as lindos at moments of extreme trauma, with the hormone triggering the regeneration itself. Recently-regenerated Time Lords can be identified by the raised levels of lindos in their system. (DWN: The Twin Dilemma, BFA: Unregenerate!)
 * At one point it was stated that Time Lords have "packets" of regeneration energy within their bodies, one for each life. These packets can be physically removed from a Time Lord's body, essentially robbing them of their regenerations. (DW: Mawdryn Undead)

The Doctor

 * First to second incarnation: Grew increasingly ill as Mondas drained his energy as well as Earth's (DW: The Tenth Planet)
 * Second to third incarnation: Enforced upon him by the Time Lords as a punishment (DW: The War Games)
 * Third to fourth incarnation: Exposure to The Great One's highly unstable web of Metebelis crystals. (DW: Planet of the Spiders)
 * Fourth to fifth incarnation: Plummeted from the Pharos Project radio telescope. (DW: Logopolis)
 * Fifth to sixth incarnation: Suffered the consequences after coming into contact with unrefined spectrox. (DW: The Caves of Androzani)
 * Sixth to seventh incarnation: Injuries sustained during The Rani's assault on The TARDIS (DW: Time and the Rani)
 * Seventh to eighth incarnation: Injured in the crossfire of a gang war and raced to hospital where the surgeons attempted to explore his unearthly physiology, only succeeding in damaging his cardiovascular system far beyond repair (DW: The TV Movie)
 * Eighth to ninth incarnation: Unknown, but involved the Last Great Time War. (IDW: The Forgotten)
 * Ninth to tenth incarnation: Cellular degeneration caused by absorbing the energies of the time vortex from Rose Tyler to both 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 channeling 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 to eleventh incarnation: Sacrificed himself to spare Wilfred Mott, absorbing all the radiation from the nuclear bolt used to power The Immortality Gate (DW: The End of Time)

On most of the above occasions, the Doctor has regenerated in the presence of other individuals (usually companions). Known exceptions include his seventh and tenth regenerations which occurred when the Doctor was alone; it is not known for certain whether his second or eighth regenerations occurred in the presence of others.

Of the Doctor's recorded regenerations to date, six have occurred with the Doctor lying in a prone position, while two (as well as the partial regeneration) occurred with him standing upright. At least four of the regenerations (first, third, sixth and seventh) occurred with the Doctor apparently unconscious (or, in one instance, already "dead"), while he appeared to remain conscious during his fourth, fifth, ninth and tenth regenerations. His first, fifth, sixth, ninth and tenth regenerations (plus the partial) occurred within the TARDIS; his third, fourth, and seventh occurred outside the ship, with his seventh regeneration the only occasion where he regenerated completely on his own without any assistance from the TARDIS or other Time Lords (The exact nature of his second and eighth regenerations remains unknown).

Exact dates are known for two of the Doctor's transformations: Seventh to Eighth (31 December 1999) and Tenth to Eleventh (1 January 2005), although in the latter case the regeneration occurs after the Doctor dematerializes the TARDIS, leaving it uncertain whether the TARDIS was still present on that date when the change occurred, especially as it proceeded to crash land near Gloucester, England in 1996 (DW: The Eleventh Hour) and the incident that killed him was December 26th 2009 (DW: The End of Time (TV story)).

Alternate Doctors

 * Ninth to tenth incarnation: Hit by Dalek fire when they tried to exterminate The Master. (DW: The Curse of Fatal Death)


 * Tenth to eleventh incarnation: Injured while trying to fix the Dalek's Zectronic beam. (DW: The Curse of Fatal Death)


 * Eleventh to twelfth incarnation: Injured while trying to fix the Dalek's Zectronic beam. (DW: The Curse of Fatal Death)


 * Twelfth to thirteenth incarnation: Hit by excess zectronic Energy, zectronic Energy was known to able to destroy the Regenerative process of a Time Lord but nevertheless the Regeneration was successful. (DW: The Curse of Fatal Death)


 * The Doctor (Unbound 3): 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. (BFA: 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)
 * Fourth to Fifth incarnation: Implied to have regenerated as a result of her involvement in The Final War in Heaven.
 * Fifth to Sixth incarnation: Regenerated a final time as she fled from Gallifrey's final destruction (possibly with K-9 Mark II in tow). Although she did manage to escape the destruction, her whereabouts are an ongoing mystery.

The Master
to be added

Rassilon
Rassilon was said to have achieved a cycle of perpetual regeneration, becoming immortal. It was for this secret that Lord President Borusa sent four of the Doctor's incarnations and their companions into the Death Zone, where Rassilon lay in eternal sleep in the Dark Tower. (DW: The Five Doctors)

Rassilon again became president in the Last Great Time War; it would seem that he was awakened from his eternal sleep and regenerated to renew himself for the war. (DW: The End of Time)

K'anpo Rimpoche
Attacked by Eight Legs and regenerated by merging with his assistant Cho Je. (DW: Planet of the Spiders)

Borusa
Borusa regenerated at least 3 times, having at least 4 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 12 regenerations.(DW: The Twin Dilemma)


 * 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)

Jenny
May have regenerated several hours after being shot by Cobb. (DW: The Doctor's Daughter)


 * However it is possible that she entered a healing coma, was restored by residual energy from The Source, or was healed by her body's residual cellular energy due to her being within 15 hours (although this is reliant on her 'birth' somehow generating extra cellular energy as a regeneration would).

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)

The Brigadier
Regenerated in 2010, which renewed his youth. (NA: Happy Endings)

Chris Cwej
Regenerated by force to survive radiation poisoning (BNA: Tears of the Oracle)

I.M. Foreman
As a priest, he had been given the gift of regenerations. This made 12 different individuals, who were created from absorbing the DNA around him. (EDA: Interference - Book One, Interference - Book Two)

K9 Mark I
Regenerated by use of a Regeneration unit after self-destructing to defeat a group of Jixen Warriors. (K9TV: Regeneration)

K9 Mark 2
Regenerated using the Regeneration unit after exhausting power core to defeat the Trojan. (K9TV: Eclipse of the Korven)

History

 * Regeneration was first introduced when the First Doctor (William Hartnell) changed into the Second Doctor (Patrick Troughton) in the closing moments of The Tenth Planet. 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 in time. (A plot device similar to this 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."
 * It is interesting to note that, despite being the Doctor's "greatest enemy", the Daleks have directly or indirectly caused less of the the Doctor's regenerations than his own race (the Time Lords). This is because 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 four regenerations depicted on screen (three featuring the Doctor in The Parting of the Ways, The End of Time and The Stolen Earth; one featuring the Master in Utopia) have taken on this form. And all four have also occurred within the TARDIS. It remains to be seen if any future on-screen regenerations will retain the same format.

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 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.
 * Russell T Davies noted how firmly the concept of limiting Time Lords to 13 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