The willingness of people to sacrifice their lives for their group has defied explanation for centuries. I suggest that identity fusion–a visceral sense of “oneness” with a group–may help understand such phenomena. In contrast to past approaches to group behavior, the identity fusion formulation emphasizes the role of the personal self and intragroup relationships in extreme pro-group action. Feelings of personal agency, perceptions of family-like ties to other group members, and a sense of group-related invulnerability mediate the link between identity fusion and pro-group behaviors.