In the long-running British science fiction television series Doctor Who, regeneration is a biological ability exhibited by the Time Lords, a race of fictional humanoids originating on the planet Gallifrey. This process allows a Time Lord to undergo a transformation into a new physical form and a somewhat different personality after instances which would normally result in death. Regeneration has been used multiple times throughout the history of the show as a device for introducing a new actor for the lead role of its main character, the Doctor. Other Time Lords and similar characters have also regenerated, usually for narrative reasons, rather than casting.

Doctor Who is a British science fiction television series broadcast by the BBC since 1963. The series, created by Sydney Newman, C. E. Webber and Donald Wilson, depicts the adventures of an extraterrestrial being called the Doctor, part of a humanoid species called Time Lords. The Doctor travels in the universe and in time using a time travelling spaceship called the TARDIS, which externally appears as a British police box. While travelling, the Doctor works to save lives and liberate oppressed peoples by combating foes. The Doctor often travels with companions.