The X-Men are a team appearing in American comic books published by Marvel Comics, first appearing in The X-Men #1 by artist/co-plotter Jack Kirby and writer/editor Stan Lee in 1963. Although initially cancelled in 1970 due to low sales, following its 1975 revival and subsequent direction under writer Chris Claremont, it became one of the most recognizable and successful franchises of Marvel Comics. They have appeared in numerous books, television shows, the 20th Century Fox X-Men films, and video games. The X-Men title may refer to the superhero team itself, the eponymous comic series, or the broader franchise including various solo titles and team books such as the New Mutants, Excalibur, and X-Force.

In the Marvel Universe, mutants are humans who are born with a genetic trait called the X-gene which grants them natural superhuman abilities. Due to their differences from the majority of humanity, mutants are subject to prejudice and discrimination and many X-Men stories feature social commentary on bigotry and justice. The X-Men have fought against a variety of enemies, including villainous mutants, human bigots, supervillains, mystical threats, extraterrestrials, and malevolent artificial intelligences. In most iterations of the team, they are led by their founder Charles "Professor X" Xavier, a powerful telepath who runs a school for mutant children out of his mansion in Westchester, New York, which secretly is also the headquarters of the X-Men. Their stories have frequently involved Magneto, a powerful mutant with control over magnetic fields, who is depicted as an old friend of and foil to Xavier, variously acting as an adversary or as an ally.

The current iteration of the official X-Men team is headquartered in The Treehouse, a Krakoan base in New York City, and the roster is voted on by their fellow mutants in elections held at periodic Hellfire Galas. No longer working in secret, they fight publicly for the safety of mutants, to build bridges between Krakoa and human nations, and to protect the Earth and Solar System from extraterrestrial threats.

From 2000 to 2020, 20th Century Fox released thirteen superhero films as part of the X-Men film series.

Marvel Studios launched the Marvel Cinematic Universe (MCU) in 2008, focused on the Avengers and their related characters, whose film rights they still owned. Marvel was then bought by Disney in 2009, but could not use the X-Men or other mutants, as their film rights still resided with 20th Century Fox. However, an alternate version of the post-credits scene in Iron Man (2008) had him specifically mention "assorted mutants" in regards to the larger universe he and Tony Stark were a part of.  Quicksilver and the Scarlet Witch were an odd case, as they had strong ties with both the Avengers and the X-Men. The studios negotiated a deal so that they could share the characters' film rights on the stipulation Marvel Studios would be unable to make reference to their background as mutants or as Magneto's children, and that 20th Century Fox could not allude to their history as Avengers members. While Pietro Maximoff / Quicksilver would only appear in two MCU films: Captain America: The Winter Soldier (2014) and Avengers: Age of Ultron (2015), Wanda Maximoff / Scarlet Witch would go on to become a semi-regular character, appearing in six films in the franchise in addition to headlining her own eponymously titled television series. On December 14, 2017, Disney announced its intent to acquire 21st Century Fox's film and television studios, which would thereby result in the film rights to the X-Men and associated characters reverting to Marvel Studios. Disney CEO Bob Iger later confirmed that the X-Men would be integrated into the MCU alongside the Fantastic Four, Silver Surfer and Deadpool. The acquisition was completed on March 20, 2019.

On July 20, 2019, during the San Diego Comic Con, Marvel Studios head Kevin Feige announced that a film centered on mutants, which will be set in the Marvel Cinematic Universe, is in development. When asked if the film will be X-Men-titled, Feige said that the terms "X-Men" and "Mutants" are interchangeable, and said that the MCU's take on the franchise will differ from 20th Century Fox's. After the deal, Charles Xavier / Professor X became the first mutant character to appear in the MCU. He appeared in the film Doctor Strange in the Multiverse of Madness (2022) as an alternate version of Professor X from Earth-838, and as the leader of the Illuminati of this universe, alongside its other members meeting Doctor Strange and putting him for trial due to his travel in the Multiverse. He is later killed by Scarlet Witch of Earth-616 while rescuing her alternate version of Earth-838. Later that year, the MCU streaming series Ms. Marvel also made reference to the X-Men; in the series finale "No Normal", Kamala Khan is told by her friend Bruno that her genetics have a "mutation", underscored by a snippet of the theme music from the 1992 X-Men series. Actress Iman Vellani, who plays Kamala, subsequently confirmed the implication that Kamala was a mutant and noted that this had been the original intention with the character in the comics.