Behind the Fantastic Four, who can name a more iconic Marvel team-up than the Uncanny X-men? Unlike the Fantastic Four, who are almost always the same four people, over the decades, the X-men have accumulated a massive roster of members and former members.
Back in the 60s, creators Jack Kirby and Stan Lee came up with a simple premise. Rather than have all their heroes suffer bizarre accidents to gain their powers, why not have them born that way? And so, the concept of mutants was born.
Following this frame of logic, the pair argued that if superheroes are born with their abilities, then they'd need some specialised school, and a teacher to instruct them on how to use their powers.
So, in the beginning, the X-Men were just that; a bunch of high school kids learning to use their abilities for good. As the decades went on, the X-Men evolved to encompass all the different aspects of the Marvel Universe, and members from every corner of the world. They're possibly the most diverse group of heroes in comic book history.
Over time, many mutants have joined the team, while others have left for greener pastures, new adventures, or just died off.