Although peers are a major influence during adolescence, the relative importance of specific mechanisms of peer influence on the development of problem behavior is not well understood. This study investigated five domains of peer influence and their relationships to adolescents’ problem and prosocial behaviors. Self-report and teacher ratings were obtained for 1787 (53 % female) urban middle school students. Peer pressure for fighting and friends’ delinquent behavior were uniquely associated with aggression, drug use and delinquent behavior. Friends’ prosocial behavior was uniquely associated with prosocial behavior. Friends’ support for fighting and friends’ support for nonviolence were not as clearly related to behavior. Findings were generally consistent across gender. This study highlights the importance of studying multiple aspects of peer influences on adolescents’ behavior.