PROJECT SUMMARY The goal of this treatment development project is to develop and pilot an adaptive mobile health (mHealth) intervention that targets the co-use of alcohol and opioids in young adults with opioid use disorder (OUD). Alcohol is an under-recognized contributor to the opioid crisis that, when used with opioids, substantially increases the risk of opioid overdose and other use-related problems. Despite clear public health significance, however, no existing intervention targets alcohol-opioid co-use. This gap is especially detrimental to young adults aged 18-25, who have the highest rates of alcohol and opioid (i.e., nonmedical prescription opioid or heroin) use and use disorders of any age group. Over half of young adults who use nonprescription opioids report past-year alcohol-opioid co-use, yet few young adults receive treatment for either substance. MHealth interventions, which use technology (e.g., smartphone applications) to intervene in daily life are highly accessible and acceptable to young adults. MHealth thus offers a means of targeting alcohol-opioid co-use that can reach young adults not currently in treatment. However, developing an effective mHealth intervention requires an ecologically valid understanding of the target behavior, which does not exist for alcohol-opioid co- use. Thus, the first step of the proposed project is to use ecological momentary assessment (EMA), wherein 60 young adults (ages 18-25) with mild to moderate OUD who report regular alcohol-opioid co-use will complete EMA multiple times daily via smartphone for 3 weeks to examine how, why, and when young adults co-use alcohol and opioids in real time in their daily lives. Using established guidelines for developing mHealth interventions, we will then use this EMA knowledge base to develop a smartphone-based mHealth platform targeting alcohol-opioid co-use. The intervention will be founded on principles of motivational enhancement therapy and cognitive behavioral therapy and deliver tailored micro-interventions based on participants’ responses to EMA assessments. We will refine the intervention based on feedback from two iterative usability studies with 10 participants. Finally, in a new sample of 60 young adults, we will conduct a Stage 1 randomized clinical trial, comparing the mHealth intervention to EMA-only and remote assessment-only control conditions. We will evaluate the intervention’s feasibility and acceptability and putative mechanisms of action. Sustained benefit will be evaluated at 3-month follow-up. This study will address national priorities to respond to the accelerating opioid crisis. It will provide vital new information on the nature of alcohol-opioid co-use in daily life and develop and pilot an innovative adaptive mHealth intervention to address this behavior in young adults.