AbstractIn the current study, we addressed several issues related to the forms (physical and relational) and functions (reactive and proactive) of aggression in community (n= 307), voluntary residential (n= 1,917), and involuntarily detained (n= 659) adolescents (ages 11–19 years). Across samples, boys self-reported more physical aggression and girls reported more relational aggression, with the exception of higher levels of both forms of aggression in detained girls. Further, few boys showed high rates of relational aggression without also showing high rates of physical aggression. In contrast, it was not uncommon for girls to show high rates of relational aggression alone, and these girls tended to also have high levels of problem behavior (e.g., delinquency) and mental health problems (e.g., emotional dysregulation and callous–unemotional traits). Finally, for physical aggression in both boys and girls, and for relational aggression in girls, there was a clear pattern of aggressive behavior that emerged from cluster analyses across samples. Two aggression clusters emerged, with one group showing moderately high reactive aggression and a second group showing both high reactive and high proactive aggression (combined group). On measures of severity (e.g., self-reported delinquency and arrests) and etiologically important variables (e.g., emotional regulation and callous–unemotional traits), the reactive aggression group was more severe than a nonaggressive cluster but less severe than the combined aggressive cluster.