Abstract Nicotine consumption in both combustible tobacco (cigarettes) and with electronic nicotine delivery systems (ENDS), or vaping, has become a serious health threat in the United States, particularly among adolescents. Decades of research have documented the many ways adolescent social networks, particularly friendship networks, influence the initiation, continued use, and brand choice of tobacco products. To date, however, there is no data or research on how friendship networks influence ENDS use. This study addresses this gap by proposing to add friendship network questions to two newly initiated cohorts of approximately 2,500 high school students each that are measuring ENDS use. Funding has already been acquired for the data collection for these cohorts and will start in November 2019 and October 2020. IRB approval to add the social network questions has been submitted to the IRB after consultation with the director of the USC office for the protection of human subjects. The friendship network questions to be added ask the students to name their closest friends in their grade at school. The names of all consented students are pre-entered into a roster so that the names auto-fill after a few letters are entered. This process has been implemented in other non-nicotine studies and works well. These data will then be used to test whether adolescents are influenced by their friends to initiate and continue ENDS use, as well as whether friends influence brand and flavor choices and marijuana uptake. In addition, network selection processes will be tested which occurs when people make network changes to be consistent with their behavior. Additional hypotheses to be tested include determining whether peer influence and selection: (1) are stronger among homophilous and/or stronger ties; (2) extends to dual- or poly-use; (3) occurs for brands and flavors choices; (4) occurs for dual ENDS and marijuana use. Given the longitudinal nature of the data we will construct ENDS and tobacco use trajectories and determine if network changes are associated with different trajectories. In sum, this proposal represents a timely opportunity to add a crucial piece of data to two newly initiated funded cohorts of school-based adolescent ENDS studies; namely, friendship network data. Preliminary data from a cross-sectional sample of 1,616 students in one school district in South Dakota showed a strong correlation between individual vaping and friend vaping. Longitudinal data are needed to determine the direction of this relationship and apply more sophisticated analytic techniques such as stochastic actor-oriented models.