ABSTRACT The context in which early drinking occurs may be an important, but neglected, determinant of progression to risky drinking behaviors and problems. Early initiation of drinking (i.e., early onset), especially of heavy drinking, has been associated with a range of adverse outcomes for adolescents, including alcohol abuse and dependence, lower academic achievement, risky sex, assaults, traffic crashes, and other substance use. The mechanisms underlying the associations between early drinking onset and problems, however, are largely unknown. These associations may reflect neurological or physiological changes resulting from early onset of drinking; the effects of common predisposing factors (e.g., genetic susceptibility, impulsivity) that increase the likelihood of both early drinking and involvement in problem behaviors; or differential exposure to and selection of drinking environments that enable and maintain heavy drinking and associated problem outcomes. Importantly, some research suggests that early initiation to drinking may be attributable to environmental, rather than genetic or other predisposing factors. No research, however, has investigated the contribution of risky contexts to the development of early drinking onset and the progression to problems. To fill this research gap, we will investigate the contexts in which early drinking, heavy drinking, and intoxication occur. We will obtain detailed characteristics of those contexts to evaluate how they relate to the later development of drinking, other substance use, and problems. The proposed study will use a mixed-method approach consisting of (a) a baseline and six follow-up surveys over 3 years with 1,500 adolescents aged 12-16 years at baseline and (b) qualitative interviews with 25 early onset drinkers recruited from the baseline survey. This longitudinal study will help to understand whether and how specific contexts contribute to maintaining alcohol use over time and the development of alcohol-related problems among early onset drinkers. This study will allow us to investigate why some youths who drink at an early age develop problems and some do not. It will be guided by a socio-ecological developmental model that emphasizes the importance of social interactions and drinking contexts for the development and maintenance of problem behaviors. The specific aims are to: (a) assess the contexts in which the first occasions of drinking, heavy drinking, and intoxication occur and how characteristics of these contexts contribute to continued drinking, heavy drinking, and problems; (b) assess whether and how drinking contexts within the first year of drinking contribute to continued drinking, heavy drinking, other substance use, co-use, and problems; (c) investigate the ongoing associations of social and other contextual characteristics, including influences of close friends, venue-based social characteristics, adult supervision, and alcohol availability, with drinking, other subs...