Objective: This study examined the relationship between ego development and psychological defenses to aggressive behavior in a group of psychiatrically hospitalized adolescents. Method: Subjects were 245 adolescents ages 12-16, who were assessed for aggressive behaviors, assaultive incidents, ego development and defenses. Results: Aggressive behavior was found to be negatively associated with ego development, regardless of age, gender or IQ. Defense styles which located the source of conflict outside of the self were associated with higher levels of self-reported aggressive behaviors. Hospital incidents of assault were associated with a lack of cognitively based defenses. Conclusion: Aggressive behavior is tied to the development of critical psychological processes which shape the adolescent's ability to understand themselves and their environment. The results are discussed in the context of a clinical-developmental approach to adolescent aggressive behavior.