As the ranks of the X-Men have grown over the decades, current and past members have become a family. And, like family, they have had disagreements. Sometimes, these have been resolved by splitting the team into different entities. Other times, it meant the departure of members for other teams in the Marvel Universe.
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The fact that nearly a dozen or more X-Men have jumped to other teams doesn't mean the family of mutants is broken. It simply means these heroes needed a fresh perspective, so they could realize how much they missed their compatriots. As an example of this, here are 10 X-Men who became members of other teams.
10 Havok (Mutant X/Avengers)
Scott Summers' younger brother, Alex, has had a long history with the X-Men. Introduced in 1969, he appeared on-and-off with them for many years. In 1991, he was made the leader to a new X-Factor team. During this time, he was cast into a parallel world. It's here that he became joined his first non-Earth-616 team -- Mutant X.
This group was comprised of darker versions of previous teammates. For instance, Ororo Munroe had both weather and vampire powers as Bloodstorm while Bobby Drake was an ice creature named Ice-Man who couldn't be touched. Later, after the events of AvX, Havok became the leader of a combined Avengers/X-Men lineup called Avengers Unity Squad.
9 Teen Cyclops (The Champions)
The Beast had the best intentions. To correct some serious issues within the present-day X-Men, he brought the original quintet back from the past. Among them was a teenage Cyclops who still believed in Charles Xavier's message of peace between mutants and humans.
As he acclimated to the new timeline, this teen Cyclops began to explore the 21st century. On top of connecting with his father once again, the young Scott Summers joined a group of teen heroes called the Champions. Over the next several months, he worked with people like Ms. Marvel, Miles Morales' Spider-Man, and Nova to help defeat new and old villains.
8 Rogue (Avengers Unity Squad)
After Professor X's death in Avengers vs. X-Men (AvX), Captain America asked former X-Man Havok to lead a new Avengers lineup called the Avengers Unity Squad. This one would feature a combination of X-Men and Avengers as a show of reconciliation between mutants and humans. Among those who joined Havok was Rogue.
Given the role of field leader, the long-time X-Man helped hone teamwork between heroes like Sunfire, Thor, and Wonder Man. She even came to terms with the Scarlet Witch, the person who caused the cataclysmic M-Day in House of M. She remained a member of the Avengers until she was called back into the X-Men's ranks a few years later.
7 Sunspot (Avengers)
Roberto de Costa, known as Sunspot, had two roles in The Avengers. The first one was as a member of the main team. Both he and his pal Sam Guthrie, aka Cannonball, became the first two members of the original New Mutants to join the team at the request of Captain America himself.
Later, Roberto took over the villainous company known as Advanced Idea Mechanics (A.I.M.) and transformed it into Avengers Idea Mechanics. Hiring a number of superheroes to help him, he created the U.SAvengers. Taking the role of a new Citizen V, he led this new group that partnered with the U.S. government but was not directly connected to S.H.I.E.L.D.
6 Prodigy (Young Avengers)
Prodigy was one of the youngest members to join the X-Men. In part because of his ability to absorb the skills of others around him. Even when he lost his powers during M-Day, he remained an integral part of the team.
When all of his knowledge was restored by the Stepford Cuckoos, he took a leave from the X-Men and landed in an adventure with the Young Avengers. Feeling more comfortable with the team, he remained a member and even started a relationship with one of his teammates.
5 Professor X (The Illuminati)
For years, conspiracy theorists have believed that a group of individuals named the Illuminati have existed as the true rulers of Earth. Well, this has now been proven ... at least on Earth-616 of the Marvel universe. However, instead of heads of industry and government leaders, it is comprised of the greatest superhero minds in the world. This included one Charles Xavier.
Prior to his demise in AvX, Xavier joined Doctor Strange, Reed Richards, and Tony Stark, among others, to mold the world and prevent it from destruction. Unfortunately, the group did more harm than good. After the "Crisis" that merged Earth-616 with the Ultimate Universe Erath, the Illuminati split up, or did they?
4 Iceman (Champions/New Defenders)
After Bobby Drake was saved from the living island of Krakoa in Giant-Size X-Men, he decided to take a break from the Merry Mutants and see what else was out there. A year later, he joined Black Widow, Hercules, Ghost Rider, and X-Men companion Angel in a new group -- The Champions.
Unfortunately, this group didn't last long, and Iceman was soon on his own. Though he did team-up with the "New" X-Men for a few adventures, he didn't officially rejoin a team until 1983 and New Defenders #125. Here, he teamed together with former teammates Angel and Beast to work with Moondragon, Gargoyle, and Defenders veteran Valkyrie.
3 Angel (Champions/New Defenders)
Warren's situation is similar to that of Iceman. Needing a break from the constant threat of death and persecution, Angel left the X-Men for a time. He joined Bobby on The Champions for a brief period, then he went solo.
At the urging of Hank McCoy, Warren joined him and Iceman on The New Defenders. He served on the team for several years. Then, with Cyclops and a rejuvenated (for the first time) Jean Grey, he, Iceman, and Beast created the new team X-Factor.
2 Wolverine (New Avengers)
Of all those who have served since Giant-Size X-Men, Wolverine is probably the most logical member who could have joined The Avengers at one point or another. Though he likes to work on his own, he is also a team player. In addition, he respects authority in most cases.
It wasn't until after the events of "Avengers Disassembled" did he agree to join a new team line-up. Led by Captain America, it featured some new and old faces. Among them another Marvel hero who was never part of the team -- Spider-Man. Since then, Wolverine has been a part of numerous Avengers line-ups.
1 Beast (Avengers/New Defenders)
Out of all the X-Men to join other teams, Beast was the first member to do so. In addition, it wasn't even due to his kidnapping on Krakoa. He wasn't even an X-Man at the time.
Shortly before Giant-Size X-Men, Hank left the team to finish his doctorate. It's during this time he became the blue and furry hero we all love. Instead of returning to the Merry Mutants, he joined new members Moondragon and Yellowjacket in Avengers #137. In doing so, he brought a lightness to the series not seen in a while.