Regeneration

The Fifth Doctor regenerates in The Caves of Androzani.

Regeneration is the process by which the Time Lords maintain their centuries-long lifespans. It is generally described as a "genetic reshuffling" which a Time Lord may go through if his or her body is worn out by age or has suffered some life-threatening injury or infection. Time Lords may even regenerate at will.

Regeneration in general

Process

Generally, regeneration occurs when a Time Lord has been too badly injured to survive.

Any ways to kill a Time Lord permanently is unknown.

Some Time Lords have a certain amount of control over the process. The Master was able to make his next regeneration as young as the Tenth Doctor (DW: Utopia) and Romana seem adept enough at the process to custom design her new appearances, such as when she went through several bodies before finally deciding on a copy of Princess Astra. (DW: Destiny of the Daleks) The process can also be stopped with some difficulty (DW: The Power of the Daleks ), as the Master did in order to spite the Doctor. (DW: Last of the Time Lords)

The actual regeneration is a painful process as bones and tissue settle into their new forms. (DW: The Power of the Daleks ) The Time Lord will also suffer from confusion and memory loss. (DW: The Twin Dilemma) During the first few hours of the regeneration, the Time Lord will be unstable mentally and physically. If they are knocked unconscious, the whole process might be started all over again. (DW: The Power of the Daleks ) 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 each regeneration there is a marked change in a Time Lord's personality. During the process of regeneration there are the genetic equivalent of 'bit errors' appear in the DNA of the regenerated cells. This is what causes the appearance of the Time Lord to change. However, because even the cells of the brain regenerate as well, their brain chemistry and organisation will change. Although the aspects of their personality caused by "nurture" will not change, the "nature" contribution to their personality will change. (source?)

Science

The exact mechanism that makes regeneration possible has not been stated, though many theories have been made. These varying explanations may or may not be compatible with each other.

• One explanation was 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 can be repaired, restored and reorganised. This will 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. Cardinal Rassilon intended for this mechanism only to be used upon the Gallifreyan elite. He also inputed a parameter of 12 regenerative cycles to avoid decaying biogenic molecules. (DW: Zagreus)

• Another theory attributes regeneration to a "nanomolecular virus" that rebuilds the body much like the "self-replicating biogenic molecules". (DW: 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. (DW: The Crystal Bucephalus)

• Another explanation is that regeneration may be linked to what is known as the "Rassilon Imprimatur", the symbiotic nuclei of a Time Lord that bonds him or her to a TARDIS, and allows his or her body to withstand the molecular stresses of time travel.

• 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. (DW: The Twin Dilemma (Novelisation), BFA: Unregenerate!)

Limitations

It was established that a Time Lord can regenerate twelve times before permanently dying, though as with most such "rules" there were occasionally exceptions. (DW:The Deadly Assassin) It is possible that the twelve-regeneration limit may be circumvented through unorthodox means. The Master, after taking over the body of a Traken, is offered "Regeneration: a whole new life cycle" as an incentive to rescue the four Doctors from the Death Zone. (DW: The Five Doctors)

Alternatively, this might just imply that a cycle of regenerations can be given to non-Time Lords.

The Doctor's arch-enemy, the Master, found his own remedy for the end of his regenerative cycle. He first attempted and failed to use the Eye of Harmony on Gallifrey to renew himself, though he was able to absorb enough energy from it to extend his life for a time. (DW: The Deadly Assassin) He was finally able to gain access to the Source on Traken, and used some its powers to steal the body of Nyssa's father Tremas. (DW: The Keeper of Traken) When this body was finally destroyed by the Daleks on Skaro, the Master's essence entered that of a human on Earth, and in this body the renegade Time Lord tried to use the powers of the Eye of Harmony's dimensional link on the Doctor's TARDIS to steal the Doctor's remaining lives. (DW: Doctor Who: The TV Movie)

Rassilon is 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) It may also have been this particular method of perpetual regeneration which Mawdryn and his team of scientists attempted to steal from the Time Lords, with the result that they became undying creatures who continually aged and regenerated until the Doctor was able to finally help them die. (DW: Mawdryn Undead) Legends suggest that Rassilon became immortal when he was turned into a vampire by the Great Vampire himself. (DW: Goth Opera)

Aborted Regeneration

Occasionally, a regeneration will fail and the regeneration will abort. 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 Lord's planet, his body attempted to regenerate. Due to the massive injuries and the lack of medical care, this regeneration aborted. The result was that he turned out looking like two individuals poorly fused together. (NA: Timewyrm: Exodus)

The Doctor's Regenerations

Following is a list of how each Doctor has regenerated, thus far.

1. First Doctor: apparently succumbs to old age and/or possibly from the energy draining effects of the planet Mondas (DW: The Tenth Planet)
2. Second Doctor: a forced regeneration and exile to Earth by the Time Lords. (DW: The War Games)
3. Third Doctor: radiation poisoning from the Great One's cave of crystals on Metebelis III. (DW: Planet of the Spiders)
4. Fourth Doctor: falls from the Pharos Project radio telescope. (DW: Logopolis)
5. Fifth Doctor: spectrox toxaemia. (DW: The Caves of Androzani)
6. Sixth Doctor: injured as The Rani attacks the Doctor's TARDIS (DW: Time and the Rani)
7. Seventh Doctor: dies on the operating table while undergoing surgery for gunshot wounds. (DW: Doctor Who: The TV Movie)
8. Eighth Doctor: unknown as of yet, possibly as a result of the Last Great Time War.
9. Ninth Doctor: cellular degeneration caused by absorbing the energies of the time vortex from Rose Tyler, which she in turn had absorbed from the heart of the TARDIS. (DW: The Parting of the Ways)

The Eighth Doctor unofficially regenerated into the Alternative Ninth Doctor.

Behind the Scenes

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 first story featuring the Second Doctor), the Doctor described himself as having been "renewed", and also said that the change was "part of the TARDIS."

The change in the Doctor's appearance was meant to occur several stories earlier, during The Celestial Toymaker, with the Toymaker capriciously changing the Doctor's appearance out of spite (the Doctor is invisible for most of the story and would have reappeared in the final episode played by a different actor). 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.

The process 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."