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.