ABSTRACT Initiation of alcohol use typically occurs well before the legal drinking age, which is concerning as early use is associated with short and long-term adverse outcomes including acute and prolonged neurobiological effects. For youth, the media constitutes a primary source of learning about alcohol. Entertainment media frequently features images of and references to alcohol, associating alcohol use with social, sexual, and financial success, with little depiction of the hazards of drinking. Moreover, alcohol content is easily accessible via new (digital) media platforms such as social media and YouTube which are highly interactive, allowing for user engagement through exchange and manipulation of information. Adolescents are high consumers of media and with the proliferation of smartphones and tablets, youth have anytime, anywhere, on-demand access to media alcohol content. Yet, virtually no information is known about in vivo exposure to such content in terms of frequency/duration, medium/format, and context. Study Aim 1 will quantify and characterize in vivo exposure to alcohol content in entertainment media (film, TV, popular music) and new media (social media, YouTube, internet, video gaming). Aim 2 will examine bi-directional prospective associations between in vivo exposure to alcohol media content and alcohol use across short-term and longer-term timeframes and Aim 3 will identify mechanisms of this association; an exploratory aim will test moderation of these associations by key individual and contextual risk factors. Etiological research points to several cognitive and social mechanisms underlying the association between in vivo media alcohol exposure and drinking, including perceived norms, cognitions (expectancies, drinker prototypes), identity, and attitudes (favorability, evaluative conditioning). Using a 3-wave measurement burst design, 300 youth age 15-18 will complete a 3-week ecological momentary assessment (EMA) protocol using a Smartphone app. Participants will be asked to provide event-level information upon encountering alcohol content and if possible, to upload a screenshot, photo, or text description of the exposure. Event surveys are paired with random-prompted EMA surveys as well as weekly surveys assessing alcohol use and longer surveys prior to each burst to measure risk factors and media utilization. Existing media intervention programs are unequipped to handle new media, fail to target adolescents, a group arguably at greatest need for intervention, and do not target media as it is experienced in vivo on portable devices. Fine- grained ecological studies such as this are needed to shape the content of just-in-time (JiT) interventions and to inform the best timing of intervention delivery, with the goal of producing reductions in underage alcohol use. In Aim 4, focus groups with a subset (n=48) of participants will provide understanding of the perceived impact of alcohol-related media content on behavior and cog...