Gender identity, or people’s deeply felt, internal sense of their gender, plays an important role in aggression perpetration and victimization. In this article, I review and organize the psychological research literatures on gender identity–based aggression. I first discuss the need to move beyond binary, cisgender understandings of gender by embracing expansive definitions that more fully capture people’s experiences and identities. Next, I summarize relevant research indicating two paths from gender identity to aggression. In one path, individuals with a more masculine (i.e., dominant, agentic) gender identity use aggression proactively, motivated by pursuit of social dominance. In another path, individuals with a more uncertain (i.e., insecure, precarious) gender identity use aggression defensively—and often toward vulnerable, gender nonconforming targets—as a means of protecting their gender identity against threats. I end by identifying important areas for future research and considering how interventions might best mitigate gender identity–based aggression.