This article examines the effect of delinquent peers on an individual's criminal activity by leveraging quasi-experimental variation in exposure to peers, separating confounding and causal effects. In particular, we examine the role of wider peer networks (i.e., classmates) as a critical source of influence on adolescents' delinquent behavior. Using a combined instrumental variables/fixed effects methodology, we address important methodological challenges in estimating peer effects. Results suggest that increasing the proportion of peers who engage in criminal activities by 5 percent will increase the likelihood an individual engages in criminal activities by 3 percentage points.