The X-Files follows the careers and personal lives of FBI Special Agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson). Mulder is a talented profiler and strong believer in the supernatural. He is also adamant about the existence of intelligent extraterrestrial life and its presence on Earth. This set of beliefs earns him the nickname "Spooky Mulder" and an assignment to a little-known department that deals with unsolved cases, the X-Files. His belief in the paranormal springs from the claimed abduction of his sister Samantha Mulder by extraterrestrials when Mulder was 12. Her abduction drives Mulder throughout most of the series. Because of this, as well as more nebulous desires for vindication and the revelation of truths kept hidden by human authorities, Mulder struggles to maintain objectivity in his investigations.

Agent Scully is a foil for Mulder in this regard. As a medical doctor and natural skeptic, Scully approaches cases with complete detachment, even when Mulder, despite his considerable training, loses his objectivity. She is partnered with Mulder initially so that she can debunk Mulder's nonconforming theories, often supplying logical, scientific explanations for the cases' apparently unexplainable phenomena. Although she is frequently able to offer scientific alternatives to Mulder's deductions, she is rarely able to refute them completely. Over the course of the series, she becomes increasingly dissatisfied with her own ability to approach the cases scientifically. After Mulder's abduction at the hands of aliens in the seventh season finale "Requiem", Scully becomes a "reluctant believer" who manages to explain the paranormal with science.

The X-Files is an American science fiction drama television series created by Chris Carter. The series revolves around Federal Bureau of Investigation (FBI) special agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson), who investigate X-Files: marginalized, unsolved cases involving paranormal phenomena. The original television series aired from September 1993 to May 2002, on Fox. 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, which took place as part of the TV series continuity, and the stand-alone film The X-Files: I Want to Believe, released in 2008, six years after the original television run had ended.

At the start of their investigations, Mulder believes in the existence of aliens and the paranormal, while Scully, a medical doctor and a skeptic, is assigned to scientifically analyze Mulder's discoveries, offer alternate rational theories to his work, and thus return him to mainstream cases. 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. They develop a close relationship which begins as a platonic friendship but becomes a romance by the end of the series. In addition to the series-spanning story arc, "monster of the week" episodes form roughly two-thirds of all episodes.

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 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 of the series.

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.

In several interviews around the release, Carter said that if the X-Files: I Want to Believe film proved successful at the box office, a third installment would be made going back to the TV series' mythology, focusing specifically on the alien invasion and colonization of Earth foretold in the ninth-season finale, due to occur on December 22, 2012. In an October 2009 interview, David Duchovny likewise said he wanted to do a 2012 X-Files movie, but did not know if he would get the chance. Anderson stated in August 2012 that a third X-Files film is "looking pretty good". As of July 2013, Fox had not approved the movie, although Carter, Spotnitz, Duchovny and Anderson expressed interest. At the New York Comic Con held October 10–13, 2013, Duchovny and Anderson reaffirmed that they and Carter were interested in making a third film, with Anderson saying, "If it takes fan encouragement to get Fox interested in that, then I guess that's what it would be."

On January 17, 2015, Fox confirmed that they were looking at the possibility of bringing The X-Files back, not as a movie, but as a limited run television season. Fox chairman Dana Walden told reporters that "conversations so far have only been logistical and are in very early stages", and that the series would only go forward if Carter, Anderson, and Duchovny were all on board, and that it was a matter of ensuring all of their timetables are open. On March 24, 2015, it was confirmed the series would return with series creator Chris Carter and lead actors David Duchovny and Gillian Anderson. It premiered on January 24, 2016. A year later, on April 20, 2017, Fox officially announced that The X-Files would be returning for an eleventh season of ten episodes, which premiered on January 3, 2018.

In January 2018, Gillian Anderson confirmed that season 11 would be her final season of The X-Files. The following month, Carter stated in an interview that he could see the show continuing without Anderson. In May 2018, Fox's co-CEO Gary Newman commented that "there are no plans to do another season at the moment."

In October 2020, Chris Carter said: "I always thought there would be even more X-Files." He admitted that continuing the series at this point with Duchovny and Anderson is unlikely, but has plans to continue the franchise with an upcoming animated spinoff. "Being that Gillian has decided to move on with her career, we certainly couldn't do Mulder and Scully again. But that's not to say there isn't another way to do The X-Files. And so right now I think the future is unwritten." The rights are now owned by Disney.