Prior research has found that proactive and reactive aggression differentially relate to many variables, including peer relations. However, no research has examined the relation between proactive and reactive aggression and peer delinquency, an important proximal predictor of adolescent antisocial behavior. The current study examined the relationship between proactive and reactive aggression and peer delinquency in a 2-year longitudinal study of 77 children ranging from 9 through 12 years of age ([X-bar] = 10.85). Of interest was whether proactive and/or reactive aggression predicted changes in peer delinquency, and whether peer delinquency predicted changes in proactive and/or reactive aggression. Results indicated that reactive aggression was associated with increases in peer delinquency and peer delinquency was associated with increases in reactive aggression, suggesting the role of both selection and socialization processes in the development of reactively aggressive behavior. Surprisingly, however, proactive aggression and peer delinquency were unrelated. Implications for the prevention and intervention of proactive and reactive aggression are discussed. (Contains 2 tables and 2 notes.)