Project Summary/Abstract Simultaneous alcohol and marijuana use (SAM; using both substances at the same time so that effects overlap) is a significant public health concern among young adults in the US. Nearly one-third of young adult drinkers report past-year SAM use and rates of SAM use are continuing to rise1. These rising rates are especially concerning, as emerging data indicate that SAM use is associated with worse short- and long-term negative consequences than alcohol-only use2-6. Thus, there is a critical need to understand psychosocial vulnerability factors related to risky SAM use. Both mean levels of negative and positive affect and affective dynamics (i.e., how emotional experiences fluctuate across time and situations) are related to risky alcohol use patterns among young adults. However, specific affective vulnerability factors associated with SAM use, and whether these factors differ from alcohol-only use, remain unknown. The proposed F32 seeks to fill this gap in the literature by conducting a secondary analysis of time-intensive daily data (MPIs: Lee/Patrick; R01AA025037) to elucidate the roles of affect and affective dynamics on SAM use outcomes among a sample of high-risk adults (N = 409). We will test whether theoretically-relevant affective dynamic processes (affective instability, negative emotion differentiation): (1) are more useful for predicting SAM use and alcohol use than mean level of affect (within- and between-subjects); (2) prospectively predict changes in SAM use and use-related problems over time; and (3) can distinguish SAM use from single substance use patterns, which has direct implications for the personalization and precision of prevention and intervention efforts. In addition to addressing these specific aims, the applicant will receive training in (1) the etiology, maintenance, and intervention of alcohol use, SAM use, and co-occurring mental health problems among young adults; (2) advanced quantitative statistics; (3) ecological momentary assessment longitudinal study development and management; and (4) professional development (e.g., manuscript and grant writing). Findings from this cost-efficient study could have important implications for prevention and intervention efforts to reduce SAM use. Specifically, personalized, technology-based interventions could incorporate emotion regulation skills training and in-the-moment information to help those at risk for SAM use to practice more adaptive strategies to manage affect.