Harry Potter

Published in the late 20th and early 21st centuries, Harry Potter was a series of novels 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)

A descendant of Rowling known as PQ Rowling wrote Harry Potter and the Half-Moon Dentist. (PROSE: Doctor Who and the Adaptation of Death) There were eleven novels. The Eighth Doctor had a complete set in his TARDIS. (PROSE: The Gallifrey Chronicles)

Martha Jones knew the books and suggested Potter's magic word "Expelliarmus" to William Shakespeare to help defeat the Carrionites. The suggestion worked, to which the Doctor cheered "Good old J.K.!!" (TV: The Shakespeare Code)

Donna Noble jokingly referred to a centaur-like Aquabi she met as Firenze — the name of one of the centaurs in the Potter universe. (AUDIO: Pest Control)

A copy of Harry Potter and the Philosopher's Stone was in a display case on Platform One. (TV: The End of the World)

Mark Whitaker owned first editions of some of the Harry Potter books. Amy Pond likened the TARDIS chasing Whitaker's train to a scene in Harry Potter, where Harry and his friend Ron Weasley chased the Hogwarts Express in a flying car. (PROSE: Touched by an Angel)

Martha Jones said she began reading Harry Potter after she read her first book series, the Troubleseekers. On a planet of fiction created from her mind, Martha and the Tenth Doctor ran through Hogwarts. (PROSE: The Mystery of the Haunted Cottage)

Behind the scenes

 * In real life, only seven Harry Potter novels (plus several shorter spin-off works) have been published. However...
 * Martha's use of Expelliarmus does come from the actual works (it's the incantation for the Disarming Charm, introduced in Chamber of Secrets.)
 * 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.
 * The fictional title Half-Moon Dentist is a play on the title of the actual sixth book, Half-Blood Prince.
 * 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.
 * According to his 2008 autobiography A Writer's Tale, Russell T 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.
 * 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.
 * In fact, David Tennant and John Hurt are the only actors to appear in Harry Potter to play the 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 TV: 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.
 * David Tennant is, just like the Tenth Doctor, a huge fan of the Harry Potter series. Tennant appeared in  as Death Eater Barty Crouch Jr. Roger Lloyd-Pack appeared in the same movie as the father of David's character. Ironically, Goblet of Fire, which includes Alastor "Mad-Eye" Moody morphing into Crouch, opened in British theaters the same day that Tennant made his first appearance in earnest as the Tenth Doctor in the Children in Need Special (TV story) also known as Born Again, which is set immediately after regeneration.
 * Coincidentally, the chapter title in the book Harry Potter and the Goblet of Fire in which Crouch Jr. receives the Dementor's Kiss is titled "The Parting of the Ways" while The Parting of the Ways is also the title of the Series 1 finale where Christopher Eccleston's Ninth Doctor regenerates into Tennant's Tenth Doctor.
 * Another odd coincide, in Harry Potter and the Goblet of Fire the trunk in which Barty Crouch Jr. kept the real Mad-Eye Moody is bigger on the inside than it is on the outside, just like the Doctor's TARDIS.