The current study examined whether proactive and reactive aggression were associated with the risk for initiation of substance use from fourth to ninth grade in a sample of 126 aggressive children (66% male). In addition, the study examined whether these functions of aggression increased risk for initiation via peer delinquency and peer rejection. Proactive aggression was marginally significantly directly associated with risk for initiation of alcohol use and indirectly associated with risk for initiation of marijuana and tobacco use through peer delinquency. Reactive aggression was associated with increased risk for initiation of tobacco and marijuana use through a complex chain that included both peer rejection and peer delinquency. However, high levels of reactive aggression that did not lead to peer rejection were negatively associated with risk for initiation of tobacco and marijuana use. Implications for intervention are discussed.