The X-Men are a team of fictional mutant superheroes appearing in American comic books published by Marvel Comics. Created by artist/co-writer Jack Kirby and writer Stan Lee, the characters first appeared in The X-Men #1 (September 1963) and formed one of the most recognizable and successful franchises of Marvel Comics, appearing in numerous books, television shows, films, and video games.

Most of the X-Men are mutants, a subspecies of humans who are born with superhuman abilities activated by the "X-Gene". The X-Men fight for peace and equality between normal humans and mutants in a world where anti-mutant bigotry is fierce and widespread. They are led by Charles Xavier, also known as Professor X, a powerful mutant telepath who can control and read minds. Their archenemy is Magneto, a powerful mutant with the ability to manipulate and control magnetic fields who leads the Brotherhood of Mutants. Both have opposing views and philosophies regarding the relationship between mutants and humans. While the former works towards peace and understanding between mutants and humans, the latter views humans as a threat and believes in taking an aggressive approach against them, though he has found himself working alongside the X-Men from time to time.

Professor X is the founder of Xavier's School for Gifted Youngsters at a location commonly called the X-Mansion, which recruits mutants from around the world to teach them how to use their powers and coexist with humanity. Located in Salem Center in Westchester County, New York, the X-Mansion is the home and training site of the X-Men. The founding five members of the X-Men who appear in The X-Men #1 (September 1963) are Angel-Archangel, Beast, Cyclops, Iceman and Jean Grey (as Marvel Girl); Professor X and Magneto also made their first appearances in The X-Men #1 and later on new members such as Havok, Polaris, Mimic and Changeling joined though only the former two managed to become a regular recurring staple of the team lineup. Eventually, the X-Men roster expanded to include a wide variety of members from numerous origins, including Wolverine, Storm, Rogue, Gambit, Jubilee, Nightcrawler, Colossus, Kitty Pryde/Shadowcat, Psylocke, Forge, Longshot, and Bishop. Since then, dozens of mutants from various countries and diverse backgrounds, and even a number of non-mutants, have held membership as X-Men.