Project Summary Up to one-third of young adults report use of marijuana or alcohol in the past month, with 8% reporting daily marijuana use and 32% engaging in heavy episodic drinking. Marijuana and alcohol use peak during emerging adulthood and can still have an impact on the developing brain. Both substances are associated with lower academic achievement, negative health effects, addiction, and driving impairment and injury. Simultaneous alcohol and marijuana (SAM) use, defined as use of both substances at the same time or in close proximation of each other with overlapping effects, is common among young adults. Past research has shown that SAM use occurs in almost one-fourth of young adults and can lead to more negative consequences than use of either substance alone. Though brief interventions have been developed for alcohol and marijuana use independently, many are targeted to college populations. Young adults in the community who are at risk for developing substance use disorders (SUDs) often have more limited options. Targeting protective behavioral strategies (PBS), such as setting limits on the frequency of SAM use, may help reduce negative consequences and prevent development of SUD. No current intervention addresses SAM use and few leverage new technological methods (e.g., smartphones) or strategies (e.g., PBS) to engage young adults. Ecological momentary interventions (EMIs) and just-in-time adaptive interventions (JITAIs) offer opportunity for interventions on SAM use by delivering intervention components in real-time, in a person's natural environment, using mobile devices. Technological advances afford increased adaptability to support an individual as his/her context changes, when the individual is receptive and in need of assistance. The goal of this treatment development project is to develop an adaptive EMI (a-EMI) that is grounded in self-regulation and social cognitive theories. To determine the most efficacious intervention strategies, we will utilize the Multiphase Optimization Strategy (MOST) framework. Historically, most interventions are built initially as a package of strategies, from which it is difficult to determine the most effective components. As an alternative, the MOST framework allows for successive testing of intervention components to identify the best combination. Following pilot testing, we will assess the feasibility and efficacy of intervention components on two outcomes (negative consequences and PBS) using a fractional factorial experimental design, with post-assessment and one- and three-month follow-ups. 136 diverse young adults recruited from primary care clinics who report current SAM use will be randomly assigned to one of eight groups, representing experimental conditions that include or do not include intervention strategies focused on craving reduction and PBS. As a result of this process, individual and/or combined components that lead to improved outcomes will be retained in a subsequent rando...