Treehouse of Horror is a series of annual Halloween-themed anthology episodes of the American animated sitcom The Simpsons. Also known as The Simpsons Halloween Specials, each episode typically consists of three separate, self-contained segments. Each segment involves the Simpson family in some comical horror, science fiction, or supernatural setting; plot elements operate beyond the show's normal continuity, with segments exaggeratedly more morbid and violent than a typical Simpsons episode. With 36 episodes as of 2025, each Treehouse of Horror episode is numbered in Roman numerals, one less than the respective season it is in; the tradition began with the second season.

The eponymous first installment "Treehouse of Horror" aired on October 25, 1990, during the second season, broadly inspired by EC Comics horror tales. In addition to parodies of horror, science fiction, and fantasy films, episodes include the recurring alien characters Kang and Kodos, unique opening sequences, and 'scary' pseudonyms in the credits. Treehouse of Horror episodes have earned high ratings and broad popularity, spawning a steady stream of merchandise, including a comic book series that ran from 1995 to 2017.

Treehouse of Horror episodes typically consist of four parts: an opening and Halloween-themed version of the credits, followed by three segments. These segments usually have a horror, science fiction or fantasy theme and quite often are parodies of films, novels, plays, television shows, Twilight Zone episodes, or old issues of EC Comics. Although they are sometimes connected by "wraparounds", the three segments rarely have any kind of continuing connection within the episode. Some have recurring elements, such as "Treehouse of Horror V", in which Groundskeeper Willie is killed by an axe in all three segments. The episodes are considered to be non-canon, which means they take place outside the normal continuity of the show.

The number of episodes of Treehouse of Horrors matches the number of series of the show: there are no such specials in season 1, two in season 34 and one in each other season. From "Treehouse of Horror" to "Treehouse of Horror XIII" and resuming with "Treehouse of Horror XXXIII", all three segments were written by different writers. In some cases there was a fourth writer who wrote the opening and wraparound segments. For the original "Treehouse of Horror", there were three different directors for the episode. From season 15's "Treehouse of Horror XIV" to season 33's "Treehouse of Horror XXXII", however, only one writer was credited with writing each Treehouse of Horror episode. "Treehouse of Horror XXXII" featured five segments. One of the season 34 Treehouse of Horror specials, "Not It", is distinguished by its uncharacteristic title and one full-length segment (divided in the two parts).

On occasion, the episodes will be used to showcase special animation, such as the "Treehouse of Horror VI" segment "Homer³", in which a computer-animated Homer is shown in a non-animated setting. At the time (1995), it was unusual for a television show to use such animation. The segment was executive producer Bill Oakley's idea and included live-action directed by David Mirkin. "Treehouse of Horror XX" included the segment "There's No Business Like Moe Business", which was the first to be musically themed.

The first, second, and fifth Treehouse of Horror episodes open with Marge standing on a stage and warning parents about the content of the episode, advising them to put their children to bed. The warning in the first episode was put in as a sincere effort to warn young viewers, as the producers felt it was somewhat scary. The entire segment was a parody of Edward Van Sloan's pre-credits warning from the 1931 film Frankenstein. Marge's warnings quickly became a burden to write, particularly because – as she herself noted – they were mostly ignored, so after "Treehouse of Horror V", they were dropped. The segment returned in the season 31 episode "Thanksgiving of Horror".

Other Treehouse of Horror episodes have opened with parodies; for example, "Treehouse of Horror III" had Homer introduce the episode in a manner similar to Alfred Hitchcock in Alfred Hitchcock Presents, "Treehouse of Horror IV" had Bart introduce the episode and segments in a manner similar to Night Gallery, and "Treehouse of Horror V" featured a parody of The Outer Limits. The sixth and seventh episodes featured short clips with no lines because the episodes had run long, and longer segments were cut. Following "Treehouse of Horror VII", the opening has been upwards of a minute long and sometimes featured an introduction by a character, such as Mr. Burns in "Treehouse of Horror XVII" or included over-the-top violence, such as "Treehouse of Horror VIII" (which showed a Fox Network censor being brutally murdered) and "Treehouse of Horror XIV" (which showed the Simpson family killing each other).

In the opening segment of the first five episodes, the camera zooms through a cemetery where tombstones with humorous epitaphs can be seen. These messages include the names of canceled shows from the previous season, deceased celebrities such as Walt Disney and Jim Morrison, and a tombstone with an inscription that read "TV violence" that was riddled with bullets as the camera panned on it. They were last used in "Treehouse of Horror V", which included a solitary tombstone with the words "Amusing Tombstones" to signal this. The tombstone gags were easy for the writers in the first episode, but like Marge's warnings, they eventually got more difficult to write, so they were abandoned. Another reason they were dropped was that the tombstones would list television shows that had been canceled the previous season; after a few years, several of the shows that were canceled were produced by former Simpsons writers. However, after two decades, this gag made a brief comeback in "Treehouse of Horror XXIX" at the very beginning, this time appearing before the main opening sequence and title.