PURPOSE:The middle school years are peak years for substance use initiation. The current study assessed the impact of peer influence on both initiation and escalation of cigarette, alcohol, and marijuana use among sixth, seventh, and eighth graders. METHODS:Youth (n = 974; 45% male) were surveyed twice over an academic year and reported on their personal substance use and their perception of peer substance use. The sample ranged in age from 10 to 15 years at Time 1 (M age = 11.95) and was 44% White, 26% Latino, 7% Asian American/Pacific Islander, 4% African American, and 14% mixed ethnic origin. RESULTS:Hierarchical regressions examined whether personal and perceived peer substance use predicted later substance involvement, and logistic regressions assessed whether Time 1 perceived peer and personal use of other substances discriminated between initiates and noninitiates. After controlling for personal substance use, perceived peer alcohol use predicted both increased alcohol and marijuana use, and perceived peer marijuana use predicted increased alcohol use. Only perceived peer alcohol use was associated with initiation of alcohol, and both perceived peer alcohol and marijuana use predicted onset of marijuana use. CONCLUSIONS:Results highlight the importance of perceived peer use in predicting both onset and escalation of use and suggest utilizing a multifaceted prevention approach that targets multiple substances.