Implementation fi delity is increasingly recognized as a key component of effective prevention programming. The present study examined the association between implementation fi delity and youth substance use outcomes among students in 11 New York City middle schools receiving a drug abuse prevention program. Trained observers monitored the implementation of a research-based prevention program by classroom teachers (N = 38), and participating students (N = 1,857) completed surveys assessing smoking and alcohol use over a 15-month period. Findings indicated that teachers who relied more on lecturing when teaching the program were less likely to use discussion and demonstration as teaching methods. Teachers who relied on lecturing were rated by observers as being less ready to teach and having poorer classroom management skills. Findings indicated that factors related to the quality of implementation signifi cantly predicted change in student substance use outcomes. Students who were taught by the most skilled providers reported signifi cantly lower increases in smoking and drinking at the follow-up assessments compared to students taught by other providers. These fi ndings suggest that teacher training to enhance implementation fi delity is a crucial component to program success in terms of student behavioral outcomes.