Harry Potter

Published in the late 20th and early 21st centuries, Harry Potter was a series of novels originated by the British author J.K. Rowling.

The Doctor was familiar with them as early as his first incarnation. (PROSE: A Big Hand for the Doctor) By his tenth incarnation, the Doctor had read the seventh Harry Potter novel, and cried upon reading the ending. (TV: The Shakespeare Code)

There were ten novels. The Eighth Doctor had a complete set in his TARDIS. (PROSE: The Gallifrey Chronicles)

A copy of Harry Potter and the Philosopher's Stone was in a display case on Platform One. (TV: The End of the World) A descendant of Rowling known as PQ Rowling wrote Harry Potter and the Half-Moon Dentist. (PROSE: Doctor Who and the Adaptation of Death)

Behind the scenes

 * In real life, only seven Harry Potter novels (plus several spin-off works) have been published.
 * Martha's use of Expelliarmus comes from the actual works (it's the incantation for the Disarming Charm, introduced in Chamber of Secrets and used throughout.)
 * The mention of Harry and Ron taking a flying car to Hogwarts also references canon, as the two of them took Mr Weasley's enchanted Ford Anglia to Hogwarts in Chamber of Secrets.
 * In 2005, Russell T Davies asked Harry Potter's creator J.K. Rowling if she would write for Doctor Who. She refused, saying that she was "amused by the suggestion but simply doesn't have the time." She was writing the sixth Harry Potter novel (Half-Blood Prince) at the time.
 * In The Christmas Invasion, a Hogwarts-style uniform featuring robes briefly appears as the Doctor considers what clothes to wear in his new incarnation.
 * The Shakespeare Code, in which the Doctor references reading the seventh book, was broadcast before Deathly Hallows was published and during a period of intense anticipation of its release. The Doctor doesn't mention the book's title, as it had not been revealed by the time the episode began filming. The fact the Doctor was emotional about the book's ending was an educated guess on behalf of the script writer that the conclusion to the long-running book series would be an emotional event.
 * According to the book The Writer's Tale by Russell T Davies and Benjamin Cook, Davies also considered basing the 2008 Christmas special around an appearance by Rowling.
 * Daniel Radcliffe, who plays Harry, was a name thrown out by Russell T Davies to play the Eleventh Doctor. In interviews in 2013, Radcliffe stated he was not interested in playing the character.
 * Other actors who have appeared in the Harry Potter films who have been considered for the role of the Doctor include Jim Broadbent, John Cleese, Robbie Coltrane, Richard Griffiths and Alan Rickman. Broadbent did play a spoof version of the Doctor in The Curse of Fatal Death, Cleese made a cameo appearance in City of Death, Griffiths was considered to play the Fourth Doctor, and Rickman was almost signed to play the Eighth Doctor by a production company before the BBC withdrew its rights in favour of co-producing the 1996 TV movie with Paul McGann.
 * David Tennant and John Hurt are the only actors to appear in Harry Potter to play the Doctor (if one does not count Broadbent's spoof turn in The Curse of Fatal Death and David Bradley's portrayal as the First Doctor).
 * When filming the 2005 series Russell T Davies commented upon the difficulty of finding little person actors for Doctor Who because Harry Potter and Charlie and the Chocolate Factory were filming at the same time.
 * Harry Melling, who plays Dudley Dursley (Harry's cousin) in the film series, is the grandson of Patrick Troughton. However, he never met him, as Patrick died before he was born. David Troughton is his uncle.
 * Michael Gambon, who appeared as Kazran Sardick in A Christmas Carol, plays Hogwarts headmaster Albus Dumbledore for most of the series.
 * Alfie Enoch, who plays Harry's friend and dorm-mate Dean Thomas, is the son of William Russell. (After the Potter series ended, Enoch adjusted his name to Alfred Enoch as he launched a TV acting career in the US.)
 * Mark Williams, who appeared in two episodes as Brian Williams, played Arthur Weasley in the Harry Potter films.
 * David Tennant is, just like the Tenth Doctor, a huge fan of the Harry Potter series. Tennant appeared in  as Death Eater Roger Lloyd Pack appeared in the same movie as the father of David's character. Coincidentally, Goblet of Fire, which includes Alastor "Mad-Eye" Moody morphing into Crouch, opened in British cinemas the same day that Tennant made his first appearance in earnest as the Tenth Doctor in the Children in Need Special minisode, which is set immediately after regeneration.
 * Both franchises are featured together in LEGO Dimensions, as is ''Harry Potters spin-off franchise '.
 * The Twelfth Doctor briefly communicated with Eddie Redmayne, who was in costume as his Fantastic Beasts character Newt Scamander, in Looking for Pudsey.