The longest-running sci-fi TV show of all time. Doctor Who can claim that title because 1) it's a damn fine piece of cult television and 2) creators have structured the series such that when members of their primary cast get old or die...or just ask for more money than the producers are inclined to pay...they are replaced by new actors and characters without second thought or mention whatsoever. Genius! The only other TV show in history able to get away with such a bait-and-switch was The Fresh Prince of Bel-Air. And really that only worked because Aunt Vivian was a minor character; it never would have flown with Uncle Phil.
As a result of Doctor Who's longevity, easy swaps, and frequent introductions, the Whoniverse has grown very large. The only failsafe way to keep track of it anymore is with a biographic directory--a Doctor Who Character Encyclopedia. Compiled in collaboration with the show's producers, the 208-page full-color hardback is filled with detailed profiles of all 11 Doctors, plus his most formidable enemies and most notable (and most hottest) companions from 1963 to present.