Crossover

A crossover character is a character who exists both within the Doctor Who universe and one or more other s. Although, within the Doctor Who Universe, fictional characters have appeared as such (within the Land of Fiction, for example), characters from other works have, from time to time, become involved.

Television
There have been two crossovers involving televised Doctor Who and other TV shows. The first was the 1993 Children in Need special Dimensions in Time, when the Doctor and some of his companions met the cast of EastEnders. This also served as part of the show's 30th anniversary celebrations.

The second, One Born Every Minute, occurred twenty years later during the 2013 Comic Relief telethon, and combined the show with characters from BBC Drama Call the Midwife.

Despite these, EastEnders is fiction in the Doctor Who Universe, as revealed in Army of Ghosts, just as characters like the Doctor and the Daleks are fictional in the continuity of EastEnders, and neither crossovers are considered to be canon.

Comics

 * When Marvel UK owned Doctor Who Monthly (later Doctor Who Magazine), characters like Death's Head, Keepsake, and the Special Executive sometimes crossed over between Marvel UK titles.
 * In 2012, IDW published Star Trek: The Next Generation/Doctor Who: Assimilation², a crossover between Doctor Who and Star Trek: The Next Generation.

Audio

 * Big Finish's Vienna Salvatori crossed over with Doctor Who in The Shadow Heart.
 * Dorian Gray appeared in the Bernice Summerfield audio Shades of Gray.
 * Both Vienna and Dorian Gray, as well as Big Finish's Sherlock Holmes, crossed over with elements of the Doctor Who universe in The Worlds of Big Finish.

Prose

 * The Old Ones originated in the, which pre-dated Doctor Who.
 * Iris Wildthyme originated in the independent works of Doctor Who novel writer Paul Magrs.
 * Sherlock Holmes and Doctor John Watson appeared as themselves in PROSE: All-Consuming Fire where they are established as real. (In the Doctor Who Universe, they do not actually use the names Sherlock Holmes and Watson, but use these as aliases.) Lord John Roxton from Sir Arthur Conan Doyle's also cameos in the same novel.
 * Carnell, the psychostrategist featured in Chris Boucher's Corpse Marker, first appeared in the Blake's 7 episode Weapon (where he was portrayed by Scott Fredericks).
 * Thomas Carnacki appeared in the Doctor Who novella Foreign Devils and the Iris Wildthyme audio Iris Rides Out.

Implicit crossovers
These usually consist of throwaway lines alluding to other fictional universes, are more common than actual appearances of characters from those universes, and are usually open to interpretation.

Audio

 * In Minuet in Hell, the Brigadier mentions the Doomwatch organization, which first appeared in.

Television

 * In Remembrance of the Daleks, Allison Williams wished that Bernard was with them, with Rachel Jensen replying that the British Rocket Group had its own problems. This is a reference to the Quatermass television serials.

Comics

 * In The Eternal Present, Theophilus Tolliver, a 19th century inventor, time traveller and friend of H.G. Wells is implied to be the protagonist of Wells' The Time Machine, who was known only as the Time Traveller (both characters have the same initials).
 * In Operation Proteus, a newspaper headline reads "Apemen At Knightsbridge". This is a reference to the TV serial Quatermass and the Pit''.
 * In Fire and Brimstone, Izzy Sinclair describes the book she's reading as featuring a city called, an old hag called , and . She asks the Doctor why they can't go somewhere like that and he replies "Izzy, I've been. It was flat." These are all references to the book series.
 * In Spam Filtered, the Doctor says he once knew a hard-light hologram who was a "bit of a jobsworth". This may be a reference to of.

Prose

 * In The Pit, mention is made of the Hob's Lane disaster. This is a reference to the events of the TV serial.
 * In White Darkness, mention is made of the Rihanssu. In the Star Trek novels, Rihannsu is the name that Romulans use for themselves.
 * In No Future, mention is made of a Soviet military group called the Vodyanoi. This is a reference to the David Wiltshire novel Child of Vodyanoi, which was adapted into the TV series.
 * In The Crystal Bucephalus, Turlough remembers reading about the effects of plasma damage on the biosphere of Qo'noS, the Klingon homeworld in Star Trek.
 * In Invasion of the Cat-People, the Doctor mentions the,  (Star Trek), ti (Larry Niven's  series) and agents of the  (Star Trek).
 * In Millennial Rites, there are brief appearances of a man in a blue cloak levitating in an old brownstone in New York, and a blond-haired man in a dirty beige trenchcoat having a Guinness in a Dublin bar. These are meant to be the comic book magicians and.
 * In The Death of Art, Anton Jarre recalls meeting a Belgian police sergeant who is clearly intended to be a young Hercule Poirot.
 * In The Dying Days, Lady Creighton-Ward (Thunderbirds), Geoffrey Hoyt and Emma Knight  appear at a party. Mention is made of a planned nuclear-waste storage facility on the Moon in two years, the first manned flight to Jupiter two years after that , MI6's double-O agents (James Bond), the IMF  and Omega Sector.
 * In The Face of the Enemy, mention is made of an English village named Little Storping. A village with the same name appears in "Murdersville", an episode of The Avengers.
 * In The Fall of Yquatine, Fitz is said to have worked at the Mother Black Cap in Camden Town in the 1960s. This is a reference to the film Withnail and I, which is set in the same time period and features a pub of the same name in the same location.
 * In Verdigris, Jo Grant reminisces about her friend Tara, with whom she'd gone to spy school and who was also assistant to an eccentric freelance gentleman adventurer in secret service to the government. This is clearly meant to be of The Avengers.
 * In The Quantum Archangel, mention is made of (from  comics), the  empire (from  comics) and Pym particles (from  comics).
 * In Eater of Wasps, when the Doctor is asked if he had ever read Tarzan, the Doctor replies that he had met him, too.
 * In Fear Itself, a character is said to possess a Stradivarius cello called the Lady Rose. A cello of this name and make appeared in the James Bond film.
 * In The Gallifrey Chronicles, a copy of the first Harry Potter novel in the Doctor's TARDIS has the first page crossed out and the words "No, no, no, no, it didn't happen like this at all" written on it in red ink, implying that the Doctor knows how it did happen.
 * In Omegamorphosis, mention is made of Gregor Samsa, who turned into an insect, and whose story was written by his friend. This is a reference to.
 * In Forever Autumn the Doctor claims to have met Jar Jar Binks' clan, and that George Lucas mistook a telepathic message for his own ideas, implying that the Star Wars stories were based on real events.
 * In The Coming of the Terraphiles, mention is made of the Conjunction of the Million Spheres, and one of the characters is named Cornelius, both connections to author Michael Moorcock's stories.
 * In Magic of the Angels, the Doctor has a large carpet bag which is bigger on the inside than the outside, and says that he is glad he got it back from Mary Poppins.