Cover-billed as 'the strangest heroes of all,' the original X-Men consisted of five teenagers still learning to control their powers:
- Cyclops (Scott Summers), who emitted powerful “optics blasts” from his eyes that could only be controlled by a "ruby quartz" visor. He would become the X-Men's field leader.
- Marvel Girl (Jean Grey), who possessed telekinetic powers and later developed telepathy.
- Angel (Warren Worthington III), who flew from two feathery wings that extended from his back.
- Beast (Hank McCoy), who possessed ape-like strength and agility.
- Iceman (Bobby Drake), who froze moisture in the air around him and who could cover his body with snow and later developed the ability to turn himself into solid ice.
Despite the philosophical concepts which appeared in X-Men, Lee has said he invented genetic 'mutants' to find a way to create a number of super-powered characters without having to come up with a separate and interesting origin for each one.
X-Men #1 also introduced the team's arch-nemesis, Magneto, who controlled magnetism and who felt that mutants should rule over normal humans. Magneto's character would later be fleshed out to reveal that he once shared a friendship with Professor X and that his decree that mutants must conquer or be conquered grew from his experiences as a Holocaust survivor. X-Men #4 introduced Magneto’s team, the Brotherhood of Evil Mutants.
While a few other important villains debuted during the 1960's — such as Professor X's superhumanly strong stepbrother the Juggernaut and the mutant-hunting robot Sentinels — the X-Men often fought easily-forgotten mutant criminals, alien invaders and brutish monsters. As a result, this era is largely regarded as unremarkable and X-Men became one of the less successful Marvel series during the 1960's.
In 1975, writer Len Wein and artist Dave Cockrum introduced a new team of X-Men. Rather than teenagers, this group consisted of adults who hailed from a variety of nations and cultures. Giant-Size X-Men #1 introduced this team, called together by Professor X to rescue the original team from captivity on a radioactive 'living island.'
This "All-New, All-Different X-Men" were led by Cyclops, and consisted of:
- Sunfire (Shiro Yashida), a hot-tempered Japanese mutant who wielded an 'atomic flame.'
- Thunderbird (John Proudstar), an Apache man who possessed super strength and speed.
- Banshee (Sean Cassidy), an Irish mutant who possessed a 'sonic scream.'
- Colossus (Piotr Rasputin), a quiet, reflexive Russian who could turn his body into 'organic steel.'
- Nightcrawler (Kurt Wagner), a rascally German who possessed great agility and the ability to teleport. Nightcrawler also had a freakish appearance including blue skin, glowing eyes and a devil-like tail.
- Storm (Ororo Munroe), a strong-willed Kenyan woman who controlled the weather. Storm would become the X-Men's leader in times of Cyclops' absence.
- Wolverine (Logan), a gruff Canadian government agent who possessed accelerated senses and a regenerative healing factor.' A covert agency had bonded the fictitious metal alloy called adamantium to Wolverine's skeleton, which included a set of three razor-sharp foot-long claws on each hand. Wolverine’s origins would become one of the series greatest mysteries.
After Giant-Sized X-Men #1, Marvel began publishing new issues of X-Men, featuring the new team (minus Sunfire and Thunderbird). The series was illustrated by Cockrum and written by Chris Claremont, who would go on to become the longest-standing contributor to the series. One of the most important storylines of this era was "The Phoenix Saga" (X-Men #101-108, 1977) in which Jean Grey (seemingly) bonded with a cosmic entity called the Phoenix and lead the team on an intergalactic mission. The saga introduced the Shi'ar alien race and its empress Lilandra, a recurring love interest of Professor X.
In 1978, Cockrum was succeeded by John Byrne, who co-plotted the series (soon retitled Uncanny X-Men). This marked the beginning of what many consider the X-Men's first creative renaissance, during which the series became one of the most popular comic books in the industry.