The fourth Doctor (Tom Baker) considers his new look in 1974
By Tim Masters
Entertainment correspondent, BBC News
He's done it on the floor. He's done it standing up. He's done it nine times already. So why does the Doctor's changing face still make the headlines?
When David Tennant's Time Lord turns into Matt Smith on New Year's Day it will be the culmination of a lengthy handover that started in October 2008, when Tennant announced he was standing down as the 10th Doctor.
Tennant, who is arguably the most popular Doctor in the programme's 46-year history, replaced Christopher Eccleston's ninth Doctor in 2005.
Smith - the youngest actor to take on the role - was cast as Tennant's replacement a year ago, and has been filming his episodes throughout 2009.
Sixties Doctors Patrick Troughton (left) and William Hartnell (right) reprised their roles in 1972 alongside Jon Pertwee (top)
When Doctor Who was conceived in the early 1960s there was no mention of the Time Lords and their ability to "regenerate" into a new person.
It wasn't until the producers wanted to replace the first Doctor (played by William Hartnell) in 1966 that they came up with the idea that set the template for most of the Doctor's future changes of face.
After his first encounter with the Cybermen in The Tenth Planet, the Doctor collapses in the Tardis and Hartnell's features are seen to change into those of Patrick Troughton - with the help of a simple vision mixing effect.
Not all the Doctors have been seen to regenerate on screen. Troughton's second Doctor was forced to change his appearance by the Time Lords in 1969's The War Games, and is last seen spinning off into a void.
It wasn't until six months later that viewers saw Jon Pertwee fall out of the Tardis wearing Troughton's clothes. It was also the first time fans had seen the Doctor on TV in colour.
Likewise, Paul McGann's eighth Doctor (from the 1996 TV movie) was never seen to turn into Christopher Eccleston's ninth Doctor when the show returned for its 21st-Century revival with Russell T Davies at the helm.
Colin Baker's sixth Doctor developed an interesting fashion sense
In the 1980s, the Doctor's regenerations largely involved him collapsed on the floor, with the faces of past companions circling round him in a torrent of special effects.
There would then be an extended sequence in which the new Doctor would parade his new eccentricities and choose his new outfit.
Eccleston's transition into David Tennant was notable because he did it standing up - with a huge burst of energy shooting out of his leather jacket.
Tennant's first words as the 10th Doctor were: "Hello. Okay. New teeth... that's weird. So where was I? Oh, that's right. Barcelona!"
As we await Matt Smith's opening line, we asked outgoing Doctor Who supremo Russell T Davies and some of the stars of David Tennant's final episode for their thoughts on why the Doctor's regeneration still seems such a big deal.
RUSSELL T DAVIES - PRODUCER/ LEAD WRITER
He's a folk hero isn't he, that's what I love about the Doctor - he transcends just a bit of TV, he's a cultural hero - grandparents remember him, parents remember him, it's a legacy that you pass on to your children.
I actually remember William Hartnell turning into Patrick Troughton and I was three years old. Of course, I remember Tom Baker turning into Peter Davison - classic stuff like that.
Peter Davison's Doctor retired to a special healing room in the Tardis to recover
It's an oral, and a visual and a family tradition that gets handed down. It is part of the family history and it is pop culture.
We put a lot of work into making it a big event because life needs big events. It's fun, no-one's dying really, no-one comes to any harm in this - it's a really joyous thing.
People who are seven years old watching this will remember this when they're 70. I think that's a lovely lovely thing.
We're not giving anything away about how he regenerates, why he regenerates or even dare I say if he regenerates - we've got a few surprises coming up...
BERNARD CRIBBINS - WILFRED MOTT
I should think a lot of it is curiosity to see what's going to happen with the next guy. I am very curious to see what Matt Smith's going to do with it. The reports I've heard - which haven't been very many - have been very, very good. And also to see how David disappears and then becomes the other guy.
[On wanting to play the Doctor] I didn't actually audition. But when Jon Pertwee was leaving, the producer Barry Letts - who died quite recently - interviewed a lot of actors, one of whom was me.
I went along and sat down and he said 'now then what can you do?' I said 'I'm a very good swimmer, I was a paratrooper, I could fight' - and he said 'Oh no, no fighting no, the Doctor is never seen fighting at all!'
So Tom Baker became the next Doctor, and one of the first things I remember him doing was knocking somebody out.
JOHN SIMM - THE MASTER
It's a big deal isn't it? - he dies and becomes somebody else and it looks quite painful. It can't be a barrel of laughs.
Mine was alright [The Master regenerated in the episode Utopia in 2007] - I came round quite quickly, I wasn't flat-out for a while. I jumped straight up and ran off with the Tardis.
Like the Doctor, his nemesis the Master has regenerated many times
It is written that Doctor Who will regenerate and change his face... now we'll have a new guy and a new angle on it, and I'm sure it will be totally different to David's. That's exactly what it has to be. I think this show is unique in that it freshens itself up every few years.
CATHERINE TATE - DONNA NOBLE
I guess it's just we're waiting for a ginger Doctor - that's pretty much what it is!
The End of Time: Part 2 is on BBC One on New Year's Day at 1840 GMT.