The Doctor's transitions:
- First Doctor (William Hartnell): Frail and steadily growing weaker throughout The Tenth Planet, the Doctor collapses inside the TARDIS at the serial's end after his first battle with the Cybermen.
- Second Doctor (Patrick Troughton): a forced "change in appearance" and exile to Earth by the Time Lords in the closing moments of The War Games.[12]
- Third Doctor (Jon Pertwee): radiation poisoning from the Great One's cave of crystals on the planet Metabilis 3 at the end of Planet of the Spiders.
- Fourth Doctor (Tom Baker): fell from the Pharos Project radio telescope in Logopolis during an altercation with the Master and was assisted in the regeneration by a mysterious "in-between" incarnation identified as "The Watcher".
- Fifth Doctor (Peter Davison): spectrox toxaemia poisoning, contracted near the start of The Caves of Androzani.
- Sixth Doctor (Colin Baker): suffered unspecified injuries when the Rani attacked the TARDIS and caused it to crash land at the start of Time and the Rani.[13]
- Seventh Doctor (Sylvester McCoy): died in San Francisco during exploratory heart surgery by a doctor unfamiliar with Time Lord physiology, after being hospitalised for non-life threatening gunshot wounds in the 1996 television movie.[14]
- Eighth Doctor (Paul McGann): unknown cause of death.[15]
- Ninth Doctor (Christopher Eccleston): cellular degeneration caused by absorbing the energies of the time vortex from Rose Tyler, which she in turn had absorbed through the heart of theTARDIS in "The Parting of the Ways".
- Tenth Doctor (David Tennant): radiation poisoning incurred while saving the life of Wilfred Mott in The End of Time. The poisoning "killed" this Doctor more slowly than other causes, allowing him to spend an unknown length of time visiting past companions.[16]