Doctor Who at 50: All the Doctor's regenerations
19 November 2013 Last updated at 08:07 GMT
Producers of the long-running series Doctor Who came up with an innovative solution to the problem of how to change the lead actor: regeneration.
The first change was in 1966 when William Hartnell transformed into Patrick Troughton with a simple mix of shots.
But throughout the following decades, regenerations became more elaborate as television production techniques advanced.
Not every regeneration was seen on screen as the series was off air for 16 years from 1989 to 2005, apart from a 1996 TV film.
And just last week, mini episode The Night of the Doctor showed an extra regeneration of eighth Doctor Paul McGann into John Hurt.
The show's producer and writer Steven Moffat told Doctor Who magazine that this does not confuse the sequence.
"He's very specific, the John Hurt Doctor, that he doesn't take the name of the Doctor. He doesn't call himself that. He's the same Time Lord, the same being as the Doctors either side of him, but he's the one who says, 'I'm not the Doctor'," he said.