Identity fusion represents a new form of alignment with groups that motivates personally costly, pro-group behaviors. The approach posits that fused individuals experience a visceral sense of “oneness” with a group, wherein their personal self (characteristics of individuals that make them unique) joins with a social self (characteristics of individuals that align them with groups). Research has identified several cognitive and affective mechanisms (e.g., sense of agency, invulnerability, familial ties) unique to fusion that help explain why strongly fused persons engage in pro-group behaviors. For example, fusion robustly predicts endorsement of self-sacrificial behaviors to save other group members' lives as well as less extreme but nonetheless personally costly acts such as donating money to needy group members. Here, we lay out the basic tenets of the fusion approach, highlight key empirical evidence for fusion theory, and discuss important issues and promising directions for future research on the topic.