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?

Previews for the new series of Doctor Who have started appearing on BBC ONE in the U.K., mixing footage from the interactive television special Attack of the Graske with segments from the forthcoming series.

You can catch the trailer in Real Media at the official Doctor Who website, in addition to some 'TARDISODES' for your mobile phone (UK only) and a look at the Sisters of Plenitude, cat-like nuns who will appear in the opening episode New Earth.

This week's Radio Times magazine also features a gate-fold sleeve with The Doctor and his assistant Rose Tyler on the front, plus numerous other supporting characters from the new series, including the return of Tom Baker's assistant Sarah Jane Smith (Elisabeth Sladen) and the robot dog K9.

The 16-page special gives enthusiasts a sneak preview of almost every episode in the new series, and some insights into David Tennant's incarnation of The Doctor (he often wears spectacles and varies the order of the buttons fastened on his suit jacket in every episode).

Check back here on Saturday night for a review of New Earth.