PROJECT SUMMARY The past decade has witnessed exponential surges in social media use among U.S. adolescents. Meanwhile, as mental health problems increase for all ages, the adverse trend is particularly striking among adolescents. Despite vigorous discussion about the contribution of social media to worsening mental health in adolescents, little objective data exist about how adolescents use social media and how it affects mental health. Few studies have examined strategies adopted by adolescents to regulate and manage their social media use; few have used detailed and objective measures of social media use and content exposure; and almost no longitudinal studies exist incorporating the strategies, social media use, and mental health. To fill these critical gaps, we propose a 2-year longitudinal mixed-method study with a national sample of 400 adolescents aged 13-17 years. Phase 1 will collect preliminary data from 100 adolescents to establish measure feasibility and optimize research procedures (Aim 1a). In Phase 2, a full-scale longitudinal data collection will be completed with 300 adolescents. Phase 1 will focus on one 14-day-long epoch where we will (1) collect daily diary reports of social media management strategies (SMMS; targeted at access, duration, timing, risky content, and risky social relations) and the social sources and approaches/tools for SMMS implementation, (2) administer baseline and end-of-epoch surveys on mental health, and (3) conduct semi-structured interviews among a subsample. During Phase 1, we will also use our existing pilot data of 154 adolescents to establish algorithms for processing data from Phase 2 (Aim 1b). Phase 2 will involve a baseline survey and 8 14-day epochs (2 years X4 quarterly epochs/year). During each epoch, in addition to the SMMS daily diary reports and end-of-epoch mental health survey, we will also use the Screenomics approach to continuously capture screenshots of adolescents' phone screens every 5 seconds to measure multiple granular objective indicators of social media use. Screenomics was IRB-approved, implemented, and proven secure and feasible in our pilot study. Annual semi-structured interviews will also be conducted among a Phase 2 subsample. The Phase 2 design will enable the examination of daily reciprocities between the multiple SMMS domains and social media use indicators (Aim 2), and the bidirectional dynamics associating SMMS, social media use, and their reciprocities with mental health (Aim 3). This highly innovative project will provide urgently needed information regarding how adolescents manage their social media use on a daily basis, which strategies influence actual social media use, and how the strategies and social media use captured through objective data influence and are influenced by mental health. Findings will offer informative solutions for healthier and safer use of social media to educators, practitioners, parents, policy makers, technology companies, and adolesce...