Christopher Eccleston is quitting his role as Doctor Who, despite the show getting the go-ahead for a new series.
Christopher, whose debut as the ninth Doctor pulled in 10.5m viewers at the weekend, said he didn't want to get stuck in one role.
His last appearance is expected to be in a Doctor Who Christmas special.
But Billie Piper, who plays the Doctor's assistant Rose, is expected to return for the new series, which was announced earlier this week.
"The audience's response to the new Doctor Who has been incredible and I'm really proud to be a part of it," said Christopher.
Doctor Who was a massive show when your parents were kids, but it's been off our screens for years - until now.
The search is now underway to find the next actor to play the time-traveller.
So who do you think should be the next 'new' Doctor Who?
Let us know using the form on the top right hand side of this page.
Who you think should play the next Doctor:
I think that Anthony Stewart Head from Buffy would be great - he is funny and has the right look.
Ed, 15, Kent
I think Ian McKellen should be then next Doctor Who!!!
William, 10, Chelmsford
I think that Jonny Depp should be the next, because he is a great actor and very funny.
Samantha, 10, Leeds
I think that that man who played Lupin in Harry Potter should be next - he is a brill actor.
Frank, 12, Birmingham
I think that Richard E Grant should play the Doctor next because he is good at playing quirky characters.
Alex, 10, Corsham
Eccleston was utterly brilliant and will be hard to replace. Shame Mark Benton can't be the Doctor now.
Mark, 13, Padstow
I think that Sean Bean (Boromir from the Lord of the Rings movies) would make an excellent 10th Doctor Who.
Steve, 15, Eastbourne
I think that Alan Cummings should be the new Doctor Who, he'd be great!
Andrew, 12, Gloucester
I think Ewan McGregor should be the new Doctor Who because he'd suit the role quite well. He's a great actor and can fit himself into any role - real-life or animated!
Sophie, 13, Peterlee