The X-Men is the most popular team of superheroes in comic books in the 1990s. Featuring an often changing lineup of young mutant superheroes and unusually complex story lines, the X-Men have found a consistently large and loyal audience of comic-book readers. Since 1980 only Spider-Man and Batman have rivaled them in popularity and sales. The X-Men's market clout has helped publisher Marvel Comics remain the undisputed industry leader, and the series' formula has been widely imitated throughout the superhero genre. Few other comic-book series of recent decades have been as influential.

Marvel first published The X-Men in 1963. The concept devised by Stan Lee and Jack Kirby was an extension of the Marvel formula already realized in such characters as Spider-Man and the Fantastic Four. The X-Men were teenaged costumed superheroes who used their powers in the service of humanity, even though the society at large misunderstood...