Recognizing that affiliating with deviant peers is one of the strongest predictors of the initiation and growth of substance use, substance use interventions often target peer relationships. Despite the strong presumption of the causal role of peers implied by intervention theory, the empirical evidence regarding peer influence is modest. To explain the modest results, more research is needed to address the question of who is influential for whom and when. It is likely that moderators at multiple levels impact the degree to which influence occurs. The current research plan applies recently developed social network analytic models to test hypotheses about individual-, peer-, network- and school-level moderators of peer influence in two samples. Exploratory hypotheses will first be developed within an intensive longitudinal study (Middle School Transitions Project) and then used to assess whether intervention participants in a second study (PROSPER) are in a position to diffuse intervention effects to non-participants. Specific aims are: (a) To identify the individual, peer, and structural characteristics of influential and at-risk youth and (b) To use the results from Aim #1 to explore whether intervention participants in a universal, family-focused substance use intervention are located in potentially influential positions within their social networks. To address these aims, children's position within the network and the characteristics of their peer groups will be identified. Then, exponential random graph models (ERGMs) will be used to test four hypotheses regarding characteristics of children and their influence in peer networks. ERGMs are an ideal method for studying peer influence, because these models provide a way to capture the interdependent and dynamic features of peer networks. By using two different samples, the current research will link developmental peer influence studies to intervention settings, where peer network dynamics may be especially important. An improved understanding of which youth are influential and which youth are susceptible to negative peer influence could improve how substance use interventions are targeted. For example, influential youth could be selected as peer leaders to promote the spread of intervention messages and steps could be taken to ensure that at-risk youth are not placed in contexts that exacerbate their risk (e.g., interventions that aggregate deviant peers). Better targeted interventions are clearly beneficial to public health, because they would be able to reduce the prevalence of substance use and its associated consequences. [unreadable] [unreadable] [unreadable]