Over the years many of the members of the respective X-Men and Avengers rosters have switched up and moving components within the Marvel Universe teams are nothing new. Sometimes characters progress to another team organically, as the narrative has been building to it or it just seems like the logical next step for the hero.
For years the X-Men and the Avengers have fought alongside each other in major calamities like the Onslaught Saga and even opposed each other on occasion as seen during Avengers Vs. X-Men.
The Avengers operate primarily from New York and although the X-Men relocated to San Francisco for a time, they've never been too far from the Big Apple, either being in Westchester or even Central Park in the recent events of X-Men Gold. Occupying the same spaces has allowed the two teams' rosters to form everything from friendships to rivalries to romances with their counterpart's members.
In order to be an official Avenger however, you need to be a certified card-carrying member, here are eight children of the atom who can officially call themselves Avengers.