Abstract E-cigarette use has increased rapidly in recent years and is now the most commonly used tobacco product among youth. Research has demonstrated that social networks (i.e. influence from friends) has an effect on adolescent tobacco use. However, no studies have examined the association between e-cigarette use and adolescent social networks over time. One possible mechanism driving e-cigarette use may be perceived friend norms, an individual’s beliefs or perceptions about the behaviors of friends and the desire to conform to these norms. However, these perceived norms are often not aligned with reality, as people often overestimate risk behaviors among friends, and actual e-cigarette use among friends may be much lower than adolescents believe. Currently, no studies have explored the interaction between perceived friend norms and friend use on individual e-cigarette use. This study will fill this research gap by analyzing friendship networks, allowing us to gain unique insight into the social dynamics influencing individual e-cigarette use. Understanding these relationships is critical for understanding the social mechanisms driving e-cigarette use among adolescents. This project will use data from two high school cohorts being recruited in current NIH-funded studies where students will complete surveys about tobacco/e-cigarette use, social network (friendship) questions, and other demographics. They will be followed throughout high school, resulting in longitudinal data on e-cigarette use and social networks. As adolescent friendship networks evolve over time, it is likely that perceived norms and patterns of e-cigarette use change as well. The longitudinal nature of the study allows us to evaluate friend influence and friend selection in relation to e-cigarette use (i.e. if people form friendships based on similar e- cigarette use behavior). Building on the previously funded R01, the objectives of this study are to (1) Assess the association between perceived peer norms, peer use, and individual e-cigarette use. (2) Evaluate the longitudinal relationship between friend influence and friend selection on e-cigarette use. (3) Use a simulation model based on collected data to demonstrate the effects of varying levels of perceived peer norms and peer e-cigarette use on individual e-cigarette use. This novel methodological approach allows us to model how hypothetical changes in perceived norms or e-cigarette use in the network would change individual e-cigarette use outcomes. The results of these analyses can be used to incorporate social networks into future interventions to change peer norm perceptions and ultimately decrease e-cigarette use among adolescents.