PROJECT SUMMARY Adolescent mental health has worsened over the last decade and increased time spent on social media is often cited as a contributing factor. Parents, policymakers, and educators are being told to curb social media use. However, whether restricting or changing exposure to social media use will impact mental health is largely unknown. This research builds on our previous prospective, longitudinal, and intensive study of a population representative sample of young adolescents. Mental health symptoms and social media use will be captured at the daily, weekly, and yearly levels via ecological momentary assessment (EMA), passive sensing, wearable technologies, and a new youth co-created open application program interface (API) toolkit. Social media engagement will be captured via real time assessments alongside the extraction of social media histories via a youth co-created API toolkit. A large representative sample of 2500 11- to 15-year-olds will be followed over four years, with a subset of 750 adolescents followed intensively via EMA to experimentally test whether social media restriction versus scaffolding (a) modifies social media engagement and (b) impacts mental health symptoms in the moment, across days, and over years. Early adolescence is a key period for testing bi-directional associations between social media use and mental health given the onset of common and costly mental disorders like anxiety and depression, evidence of heightened response to interventions, and the fact that young people begin to navigate online spaces independently at this time. The study is positioned to impact on science, practice, and policy by (1) advancing discovery related to differential bi-directional influences between social media and mental health, (2) testing whether experimental modifications to social media-engagement impact same day and future mental health symptoms, (3) identifying subgroups of adolescents for which bi-directional linkages and/or intervention impacts may be amplified, and (4) creating a novel resource for the field that will allow adolescents to access, control, and share their digital trace data to advance research, interventions, and policy.