Alcohol?s ability to improve mood in the face of stress is among its most prized reinforcing properties, long held by researchers to be of critical importance for the understanding of the etiology of alcohol use disorder (AUD). Drinkers overwhelmingly report that the mood-enhancement they gain from alcohol is most pronounced in the context of stress, and individuals who report higher levels of stress relief from alcohol are at risk for developing AUD. But attempts to capture this key element of alcohol reinforcement using experimental methods have yielded strikingly inconsistent results. One remarkable feature of this prior experimental research is that, in attempting to capture alcohol?s stress-relieving effects, researchers have strayed far outside the range of stressors typically encountered in typical drinking contexts, contexts that are overwhelmingly social in nature. In the proposed study, we revisit these critical questions using a novel approach, looking to the settings in which people actually drink alcohol to identify manipulations capable of capturing alcohol stress-relief. More specifically, we explore two of the most fundamental social stressors?lack of social familiarity and poor relationship quality?as moderators of alcohol reinforcement. We examine whether alcohol is more reinforcing within the context of unfamiliar vs. familiar social interaction and, further, whether alcohol is more reinforcing within the context of low vs. high quality social relationships. Capturing alcohol?s effects within the context of natural social discourse presents formidable methodological challenges, and we bring multiple methods to bear on our questions, combining experimental and ambulatory methods and employing state-of-the-science measures of emotion and alcohol consumption. We examine our study aims in a sample of individuals with high relevance for the understanding of AUD etiology?young heavy social drinkers. In the laboratory arm of the study, 360 individuals will be randomly assigned to consume either a moderate dose of alcohol or a control beverage in the company of either familiar or unfamiliar individuals. Of these individuals, 180 will also participate in a 21-day ambulatory assessment period to examine the interaction of alcohol and social stress in daily life. In the ambulatory study arm, participants will wear transdermal ankle bracelets to assess BAC (calibrated for each individual within the laboratory session) and will further provide information about their mood and their social contexts in response to random prompts. The Facial Action Coding System, a comprehensive, anatomically-based system for categorizing facial muscle movement, will be employed to assess emotion in both laboratory and ambulatory arms of the study. The proposed research contributes to the understanding of AUD by addressing one of the most fundamental questions in the alcohol literature?the question of why people drink. In addition, by looking to ?real life? to identify stressors, the proposed study might have a variety of key implications for AUD etiology and interventions, including for harm-reduction and prevention programs, reducing rates of relapse, and the identification of those at risk for AUD.