PROJECT SUMMARY/ABSTRACT Taken together, alcohol and drug use account for more morbidity and mortality than any single disease or disorder in the United States. Rates of substance use are highest among older adolescents and younger adults, which are critical developmental windows for the development of addiction. The combined factors of sensation seeking versus behavioral and cognitive control are underdeveloped in adolescence. Under popular models of adolescent use, such as the dual systems model, these two systems are hypothesized to explain developmental trends in adolescent substance use. More recently, such theories have incorporated the potential impact of social context, including deviant peers, as accentuating sensation seeking and reward systems in adolescents and contributing to their increased risk of substance use and dependence. To date, these developmental models have been tested and characterized primarily in cross-sectional studies or longitudinal studies with, at best, annual assessments. Here, we will use smartphone sensors and weekly surveys to assess substance use, executive function, disinhibition, risk-taking, and social context on a quasi-continuous basis over the course of multiple years in a large sample during the transition from adolescence to young adulthood. The results will provide a fine-grained model of developmental change in key risk domains and their relationship to substance use. Instrument variables derived from the smartphone's GPS, camera, and microphone, combined with an adolescent twin study design, will provide stringent tests of whether and how environmental and social context disrupts normative developmental trends.