Previous research has documented the consequences of feeling fused with a group; here we examine the nature of identity fusion. Specifically, we sought to determine what fusion is and the mediating mechanisms that lead fused individuals to make extraordinary sacrifices for their group. Guided by the assumption that fusion emphasizes the extent to which people develop relational ties to the group, we developed a measure designed to capture feelings of connectedness and reciprocal strength with the group. In 10 studies, the newly developed scale displayed predicted relationships with related measures, including an earlier (pictorial) measure of fusion and a measure of group identification. Also as expected, fusion scores were independent of several measures of personality and identity. Moreover, the scale predicted endorsement of extreme progroup behaviors with greater fidelity than did an earlier pictorial measure of identity fusion, which was, in turn, superior to a measure of group identification. Earlier evidence that the personal and social selves of fused persons are functionally equivalent was replicated, and it was shown that feelings of agency and invulnerability mediated the effects of fusion on extreme behavior. Finally, Spanish- and English-language versions of the verbal fusion scale showed similar factor structure as well as evidence of convergent, discriminant, and predictive validity in samples of Spaniards and Americans, as well as immigrants from 22 different countries. This work advances a new perspective on the interplay between social and personal identity.