The Simpsons Treehouse of Horror episodes, also known as The Simpsons Halloween episodes, are a series of episodes in the animated television series The Simpsons. They are Halloween specials, each consisting of three separate, self-contained segments. These segments usually involve the Simpson family in some horror, science fiction, or supernatural setting. Considered non-canon, they always take place outside the normal continuity of the show and completely abandon any pretense of being realistic. The first Treehouse of Horror episode aired on October 25, 1990 as part of the second season and was inspired by EC Comics horror tales. As of 2007, there are 18 Treehouse of Horror episodes, with one airing every year. The episodes are known for being far more violent and much darker than an average Simpsons episode. Episodes contain several trademarks, including the alien characters Kang and Kodos, "scary names" in the credits, a special version of the opening sequence, and parodies of horror and science fiction films. The show's staff regard the Treehouse of Horror as being particularly difficult to produce as the scripts often go through many rewrites, and the animators typically have to design new characters and backgrounds.