On Monday, Peter Capaldi told BBC Radio that the upcoming season of Doctor Who, set to premiere on April 15 on BBC America, would be his last playing the title role. We take for granted how lead actor turnover is standard operating procedure for Doctor Who, but it's a unique feature of the show that has allowed it to survive, off and on, for more than 50 years.

The key moment came in 1966 when the original Doctor, William Hartnell, had to leave the highly successful BBC show because of health problems. Showrunners Innes Lloyd and Gerry Davis, facing a casting problem that would normally kill a new series, realized that since the Doctor was an alien, Hartnell's departure didn't mean they had to kill off the main character. Instead of dying, the Doctor would "regenerate" into a different actor, and the series would simply move on.

But that wasn't all. Lloyd and Davis also allowed the actor playing the next Doctor, Patrick Troughton, to put his own spin on the performance, imbuing the Doctor with new character quirks and attitudes. Every other actor since then — there have been 13, including Capaldi — has followed suit. Some things about the Doctor never change, like his preference for wits over violence, along with a charming oddness. He also remembers everything from previous incarnations, giving the show much-needed continuity (which helps keep viewers loyal).

Otherwise, each Doctor is as different from the one before as the actor wants and the show's writers will allow. Instead of 12 impressions of William Hartnell, we get 13 distinct Doctors, each one evolving to suit the needs of contemporary audiences.

Since Capaldi's departure means we'll hopefully get another fresh take on the Doctor, let's take a look back at how the concept of "regeneration" has been tweaked over the years and what that means for Doctor Who's future.



1. First Doctor - William Hartnell

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Hartnell was already a veteran of the British theatre and several films when he took on the role of the Doctor at the age of 55 in 1963. Doctor Who was massively successful, hitting as many as 12 million viewers — the kind of numbers only Monday Night Football receives in the modern TV landscape. Unfortunately, Hartnell had serious heart problems that interfered with his performance on set. Hartnell's final appearance was in 1972’s "The Three Doctors," and he died in 1975.

2. Second Doctor - Patrick Troughton

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Troughton is arguably the biggest influence on later performances of the Doctor, if only because his playful take was such a departure from Hartnell’s more serious, paternal approach. Troughton was the first to demonstrate how other actors could always make the role their own.

3. Third Doctor - Jon Pertwee

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When Jon Pertwee was brought on to play the Third Doctor, the story was that the Doctor was being punished for meddling in the affairs of the universe with a new regeneration and exile on Earth. Conveniently for the show’s design budget, the Doctor was exiled in England during the early 1970s and worked as a science advisor to UNIT, the international alien-fighting military organization that had first appeared during the Troughton years. When the Third Doctor dies in the episode "Planet of the Spiders," a character uses the word "regenerate" for the first time to describe the Time Lord's rebirth.

4. Fourth Doctor - Tom Baker

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It was during Tom Baker's run — specifically in 1976's "The Deadly Assassin" — that the series established that Time Lords can only regenerate 12 times for a total of 13 incarnations. The topic of regeneration limits comes up because the Doctor realizes that his perennial nemesis, The Master (another Time Lord), is on his final cycle and looking to extend his life. (Spoiler Alert: The Master is still alive and kicking, now in a female form calling herself 'Missy.')

5. Fifth Doctor - Peter Davison

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When the Fifth Doctor succumbs to poison in "The Caves of Androzani," he worries that he might not be able to regenerate, asking, "Is this death?" He only manages to pull through when he sees visions of previous companions urging him to keep going. The scene most likely intended to heighten the tension, since viewers by now expected the Doctor to regenerate into the next actor, but it did leave open the possibility that we might not get to see all 13 Doctors before the character's eventual death.

6. Sixth Doctor - Colin Baker

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Until the Sixth Doctor, Whovians always saw the Doctor regenerate at the end of their run. But the BBC was unhappy with Colin Baker's take on the role and executives fired him after three years. Understandably miffed, he refused to film a regeneration sequence.

7. Seventh Doctor - Sylvester McCoy

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Sylvester McCoy opened his time as the Seventh Doctor by playing the Sixth — that's actually McCoy in the Sixth Doctor's regeneration wearing a blonde wig to mimic Colin Baker's look. The BBC cancelled Doctor Who in 1989, and in the last televised episode the Seventh Doctor simply walks off. McCoy did return, however, in the 1996 Doctor Who TV movie. He steps out of the TARDIS and is almost instantly shot by San Francisco gang members (notably, he's actually killed when surgeons try to patch up his alien body). Later in the city morgue, McCoy regenerates into Paul McGann.

8. Eighth Doctor - Paul McGann

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Doctor Who: The Movie was meant to re-launch the TV series after a seven-year hiatus but it didn't quite pick up steam the way the producers had hoped. Since McGann was supposed to jump to the series, the Eighth Doctor survives the movie. In fact, we didn’t see his regeneration until a seven-minute webisode released in 2013 for Doctor Who's 50th anniversary called "The Night of the Doctor." When Eight regenerates after dying in a spaceship crash, he finds himself talking to the Sisterhood of Karn (who first appeared on the show way back in 1976). They tell him that he can select his next form: "Fat or thin? Young or old? Man or woman?" This a rare occasion where the Doctor has any kind of choice over his next look — generally he's surprised by his new face and body. In this case, the Doctor regenerates into a very young-looking version (thanks to computer effects) of John Hurt.

9. The War Doctor - John Hurt

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Here's where we jump the rails a bit and regenerations no longer line up with the numbered Doctors. That's because the Doctor didn't consider this incarnation of himself to be a good person, therefore he's not a 'Doctor.' The War Doctor only plays a major role in the 50th anniversary movie, "The Day of the Doctor." With no apparent injuries, the War Doctor seems to simply die of old age (as did the First Doctor), saying, "I suppose it makes sense, wearing a bit thin."

10. Ninth Doctor - Christopher Eccleston

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We meet the Ninth Doctor presumably some time after his regeneration from the War Doctor. He mentions that the Daleks are all dead and he's the only living Time Lord. When Nine regenerates in "The Parting of Ways," he mentions that his next form could have two heads or no head at all — a fun thought, but it's hard to imagine an actor (or two) signing up for a two-headed character for several years.

11. Tenth Doctor - David Tennant

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In the 2008 episodes "The Stolen Earth" and "Journey’s End," a Dalek fatally shoots the Doctor, and he begins regenerating. Instead of morphing into a new actor (Tennant wouldn't hand off the role until the 2010 episode "The End of Time"), the Doctor halts the process and simply heals himself. Then he funnels the rest of his regeneration energy into his own hand that had been severed in a previous episode, creating a human/Time Lord hybrid clone of himself, along with imbuing his human companion Donna Noble with the knowledge of a Time Lord. The result of this entire process was that the Tenth Doctor had burned through one of his regenerations. That meant the Eleventh Doctor would be his last incarnation. Or so we were led to believe ...

12. Eleventh Doctor - Matt Smith

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In the 2013 Christmas special "The Time of the Doctor," the Doctor (a prosthetically aged-up Matt Smith) seems to be finally facing his death after centuries of fighting the Daleks. But after his companion Clara begs the Time Lords for help, they grant the Doctor an entirely new cycle of regenerations. Thanks to that deus ex machina, Doctor Who can keep casting new actors or actresses to play the Doctor for at least another 50 years.

13. Twelfth Doctor - Peter Capaldi

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Right now there's some ambiguity over just how many regenerations the Doctor's new cycle could entail. The reality is it depends on the needs of one script or another down the line and how writers decide to raise the stakes. But it's worth mentioning that in "Kill the Moon," the Twelfth Doctor says offhandedly that he's not sure how many regenerations he has left. And in "Hell Bent," the most recent season finale, even Rassilon, the leader of the Time Lords, asks, "How many regenerations did we grant you?" Of course, only time will tell.