This study tested whether proactive and reactive aggression were differently predictive of later externalizing problems such as delinquency and DSM-related disruptive behaviors (i.e. oppositional defiant and conduct disorders). It also tested whether these two subtypes of aggressive behaviors interacted in predicting externalizing problems. A community sample of low SES boys participated in the study. Proactive and reactive aggression were rated by teachers when boys were 12 years old. Delinquency, oppositional disorders, and conduct disorders were assessed during mid-adolescence. Proactive but not reactive aggression predicted delinquency and disruptive behaviors. Moreover, high levels of reactive aggression weakened the link between proactive aggression and delinquency. Reactive aggression, however, did not moderate the link between proactive aggression and disruptive behaviors. We conclude that reactive and proactive aggression are two distinct types of aggressive behaviors with different predictive abilities. We also offer tentative explanations to account for the present findings.