The X-Files is an American science fiction drama television series created by Chris Carter. The original television series aired from September 1993 to May 2002 on Fox. During its original run, the program spanned nine seasons, with 202 episodes. A short tenth season consisting of six episodes ran from January to February 2016. Following the ratings success of this revival, The X-Files returned for an eleventh season of ten episodes, which ran from January to March 2018. In addition to the television series, two feature films have been released: The 1998 film The X-Files and the stand-alone film The X-Files: I Want to Believe, released in 2008, six years after the original television run had ended.

The series revolves around Federal Bureau of Investigation (FBI) Special Agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson), who investigate the eponymous "X-Files": marginalized, unsolved cases involving paranormal phenomena. Mulder is a skilled criminal profiler, an ardent supernaturalist, and a conspiracy theorist who believes in the existence of the paranormal, whereas Scully is a medical doctor and a skeptic who has been assigned to scientifically analyze Mulder's discoveries. Early in the series, both agents become pawns in a larger conflict and come to trust only each other and a few select people. The agents also discover an agenda of the government to keep secret the existence of extraterrestrial life. Mulder and Scully's shared adventures lead them to develop a close platonic bond, which by series' end develops into a romantic relationship. Roughly one third of the series' episodes follow a complex story arc about a planned alien invasion, whereas the other two-thirds are "monster of the week" episodes that each focus on a one-off villain, mutant, or monster.

The X-Files was inspired by earlier television series which featured elements of suspense and speculative fiction, including The Twilight Zone, Night Gallery, Tales from the Darkside, Twin Peaks, and especially Kolchak: The Night Stalker. When creating the main characters, Carter sought to reverse gender stereotypes by making Mulder a believer and Scully a skeptic. The first seven seasons featured Duchovny and Anderson equally. In the eighth and ninth seasons, Anderson took precedence while Duchovny appeared intermittently. New main characters were introduced: FBI Special Agents John Doggett (Robert Patrick) and Monica Reyes (Annabeth Gish). Mulder and Scully's boss, Assistant Director Walter Skinner (Mitch Pileggi), also became a main character. The first five seasons of The X-Files were filmed and produced in Vancouver, British Columbia, before production eventually moved to Los Angeles to accommodate Duchovny. The series later returned to Vancouver to film The X-Files: I Want to Believe as well as the tenth and eleventh seasons.

The X-Files was a hit for the Fox network and received largely positive reviews, although its long-term story arc was criticized near the conclusion. Initially considered a cult series, it turned into a pop culture touchstone that tapped into public mistrust of governments and large institutions and embraced conspiracy theories and spirituality. Both the series itself and lead actors Duchovny and Anderson received multiple awards and nominations, and by its conclusion the show was the longest-running science fiction series in U.S. television history. The series also spawned a franchise that includes Millennium and The Lone Gunmen spin-offs, two theatrical films, and accompanying merchandise.