Doctor Who

 is a British science fiction television series that depicts the adventures of a time-traveling alien known only as "The Doctor". Originally premiering in 1963, it is listed in the Guinness Book of World Records as the longest running science fiction television series in history though with breaks in production.

In The Big Bang Theory

 * Sheldon's Saturday morning ritual includes getting up at 6:15 am and watching Doctor Who on BBC America. This was once disrupted when Penny crashed out on the guys' couch.
 * A replica of a Dalek (one of the Doctor's most famous adversaries) is sometimes visible on one of the shelves in Leonard and Sheldon's living room.
 * In some shots of the comic book store, a poster of an exploding TARDIS appears on the wall. This is a replica of a painting, "The Pandorica Opens," which in the Doctor Who mythology was painted by . The painting figures prominently in the two-part series 5 finale, "The Pandorica Opens" and "The Big Bang."
 * In the season 4 episode "The Justice League Recombination," Stuart wears the costume of the Fourth Doctor (one of the most famous incarnations, played by Tom Baker) during the New Year's Eve party.
 * In the season 5 episode "The Beta Test Initiation," Sheldon, Leonard, Penny and Amy are watching an episode of Doctor Who (either "Silence in the Library" or "Forest of the Dead," both of which take place during the time of the tenth incarnation of the Doctor, played by David Tennant) at Sheldon and Leonard's apartment. When Penny states that her idea of a date with Leonard was not watching Doctor Who and Amy notes the Doctor's fondness for contemporary London, Sheldon kicks Penny out of going to Who-Con and threatens to do the same to Amy.
 * In the season 7 episode "The Convention Conundrum," Sheldon reacts to not going to Comic-Con by attempting to hold his own convention. Matt Smith, who played the Eleventh Doctor in series 5 through 7, is on his list of science fiction icons to invite.
 * In the season 6 episode "The Holographic Excitation," Sheldon's list of couples costumes he likes includes "The Doctor & A Dalek."
 * In the same episode, Raj gets a TARDIS photo booth for Stuart's Halloween party, leading Sheldon to call it a "major rager." Later, Leonard (as Doctor Albert Einstein) "explains the theory of relativity" to Penny (who is dressed as a sexy cop) in the photo booth (twice!).
 * Penny's costume may be a reference to (A) Eleventh Doctor companion Amy Pond (played by Karen Gillan), who wore a "sexy cop" costume in her first episode, "The Eleventh Hour," or (B) the TARDIS itself, which according to the episode "The Doctor's Wife" likes to be known as "sexy."
 * In the season 7 episode "The Romance Resonance," a line in the song "If I Didn't Have You" contains the lyrics "I'd be Doctor Who without the TARDIS." Later, Leonard can be seen wearing a t-shirt featuring the Seal of Rassilon, a Doctor Who motif.
 * In the season 7 episode "The Proton Transmogrification," Sheldon tells Professor Proton in his dream that he's had to say goodbye to eleven Doctor Whos. This refers to the times that the Doctor has "regenerated," a process that restores his body to health but completely alters his appearance and personality (developed by the producers in 1966 as a way to replace First Doctor actor William Hartnell with Second Doctor actor Patrick Troughton, and subsequently used whenever the actor playing the Doctor leaves the show). The most current regeneration was from Matt Smith's Eleventh Doctor to Peter Capaldi's Twelfth Doctor in the 2013 Christmas episode "The Time of the Doctor."

Doctor Who