With more than 700 episodes, Doctor Who is the longest running science-fiction series of all time, says the Guinness Book of World Records. Long ago, the series hit on the clever idea of creating a sci-fi explanation that would allow the Doctor to appear in a various "incarnations" therefore neatly explaining away periodic changes in actors, and allowing the series to remain fresh. In fact, in its twenty-first century version, which followed a near-decade break in production from 1996 until Christopher Eccleston picked up the role in 2005, The Doctor seems as popular as ever.

Doctor Who first appeared on the BBC in 1963. "The Time Lord" has been played by ten actors in all. David Tennant now has the role, and season two episodes of the new Doctor Who will appear in the U.S. on the SciFi Channel beginning tonight, with "New Earth." Billy Piper is still along as plucky Rose.

Who's your favorite Doctor?