Regeneration has been a part of Doctor Who ever since the first actor in the role, William Hartnell, became too old to continue as the show's main lead. Originally conceived as a way to hand wave away the introduction of a new actor in the title role, regeneration is a plot device that has allowed Doctor Who to become one of the most enduring sci-fi TV shows of all time. Whenever the current actor playing the role of The Doctor wants to leave the show, the writers can simply regenerate the character and bring in a new actor in their place.