ABSTRACT Alcohol-related risks are highest in young adulthood and this age group is most likely to be exposed to the alcohol use of their peers. Young adults are also highly influenced by their peers, both for high-risk behaviors and for prosocial actions. Bystander helping is one way that peer influence in potentially dangerous alcohol consumption circumstances can reduce harm to those at risk. Bystander helping is the phenomenon when witnesses to a problematic event step in to intervene in some way, but bystander helping has been under- investigated when hazardous alcohol use is the target behavior. In order to develop interventions that effectively leverage peer influence for harm reduction in the young adult community, research must be conducted on the circumstances of alcohol-related events, and specifically the experiences of those who could be bystander helpers. Drawing from bystander intervention research on sexual assault and bullying prevention, we propose to investigate the contexts and conditions under which bystander helping for hazardous alcohol use occurs naturalistically. The aims of this research are to investigate the event-level (Aim 1) and individual difference (Aim 2) predictors of bystander helping in response to peer alcohol-related risk, and to investigate the relationship between helping approaches and bystander and peer outcomes (Aim 3). Following community engagement with an advisory group and a pilot phase in which we will finalize procedures and assessments, we will conduct an assessment study with a sample of young adults (N = 200; ages 18-25) balanced on gender and reflecting the US in race/ethnicity, who will report on their exposure to the hazardous drinking of others in their environment for 28 days using ecological momentary assessment (EMA) methods. Participants will complete random and morning reports on their own smartphones that will collect information about alcohol- related behavior witnessed, the social context, the characteristics of the person showing the risk behavior, the nature of the relationship between that person and the bystander, the bystander’s own state including their own level of intoxication, perceived barriers to intervention, bystander strategies used, outcomes of any bystander helping, and the outcomes to the person showing the hazardous behavior. Each evening survey will pull forward information from the previous survey to allow for assessment of subsequent observations, and morning surveys will assess further the risks observed, behavioral responses, and outcomes. Methods used during the baseline assessment and orientation, including a social network interview, will facilitate brief, accurate and private EMA reports. We expect the information derived from this investigation will contribute in a substantive way to the development of effective trainings for individuals who are exposed to the hazardous drinking of others.