ABSTRACT Opportunities and constraints to drink in different contexts change when young people can access alcohol legally, but also change momentarily across nighttime hours. We will investigate how contextual risks (e.g., drinking locations, number of people, other's drinking) across weekend evening hours change when young people attain the legal drinking age and how they contribute to heavier drinking and alcohol-related problems. Young people drink most often during weekend nighttime hours. Prior research has demonstrated that over those hours drinking and problems may increase, and that drinking is associated with situational (e.g., alcohol availability) and social (e.g., number of people, age composition) characteristics of drinking locations. The legal drinking age is a life milestone that affects young people's drinking contexts, behaviors, and problems. Our goal is to identify the contextual and personal characteristics (e.g., drinking motives) that contribute to drinking risks over evening hours to inform nighttime preventive interventions for young people moving to the legal drinking age. The specific aims are to: (1) measure changes in use of contexts and context-specific risks before and after the legal drinking age, (2) assess differences in personal and contextual covariates related to the use of nighttime drinking contexts and transitions between contexts among drinkers before and after the legal drinking age, (3) determine the degree to which personal and contextual covariates differently increase risks for heavier drinking and problems over evening hours before and after the legal drinking age, and (4) evaluate the roles of evening plans on actual drinking and problems over evening hours. We propose a two- wave Ecological Momentary Assessments (EMA) study with a sample of 300 young adult current drinkers in California before they are of legal drinking age (19-20 years old) and two years later, after they attain legal drinking age (21-22 years). A baseline and three 6-month follow-up surveys between EMA waves will be used to assess and track changes in personal covariates (e.g., prior drinking, college attendance) and maintain a high response rate. EMA is well suited to examining changes in contexts, opportunities, constraints, drinking behaviors, and risks as they unfold in natural settings over evening hours because it minimizes recall bias, maximizes ecological validity, and allows assessment of micro-ecological processes that influence behaviors and risks in near real time. Our sample will include college and non-college young adult drinkers in different life circumstances (e.g., living on their own or with parents) to capture the full range of drinking contexts and opportunities of young adults as they move to legal drinking age. Results will inform context-based nighttime preventive interventions by specifying when, where, and to whom prevention options should be provided.