PROJECT SUMMARY/ABSTRACT Emerging evidence indicates that co-use of cannabis and e-cigarettes may perpetuate dependence on both products and lead to adverse health outcomes in young people. Vulnerable groups, such as the Native Hawaiian, Pacific Islander, and Filipino (NHPIF) young adults, often use these products at higher rates than other racial/ethnic groups. However, substance use among NHPIFs has often been overlooked because of the tendency among researchers to treat Asian American, Native Hawaiian and Pacific Islanders (AANHPIs) as a monolithic group. Although AANHPIs are often regarded as a “low risk” group, when disaggregated, AANHPI subgroups show marked health disparities, with NHPIFs indicating disproportionately higher rates of tobacco and other substance use and higher burden of mortality due to cancer and cardiovascular diseases. Thus, there is an urgent need to examine the unique risk factors that drive higher substance use among NHPIF and rapidly translate findings into scalable interventions. Despite e-cigarette and cannabis co-use being common, it has not been studied in NHPIFs. The current phased study will utilize epidemiological data from two sources (R61 phase) – the cross-sectional Behavioral Risk Factor Surveillance System (BRFSS) and the longitudinal Young Adult Health Behavior Survey (YAHB) – to guide the development of a Just-in-Time Adaptive Intervention (JITAI) for e-cigarette and cannabis co-use (R33) among NHPIF young adults ages 18-30. JITAIs allow for intervention components to be delivered in real-time through a smartphone application. In the R61 phase, we will identify the relative importance of various social determinants of health in predicting co-use among NHPIF young adults. Across both surveys, we will examine associations between e-cigarette and/or cannabis use, rates (prevalence and proportion of use), patterns, and progression (initiation, escalation, and maintenance), with sociocultural (social vulnerability, family/peer influence and substance use), community, and advertising/marketing factors, by ethnicity. We hypothesize that low-SES NHPIFs will be identified as the most vulnerable group for co-use and that social influences will be most highly associated with use, initiation, escalation, and maintenance. Findings will allow us to calibrate the proposed intervention and tailor the balance of JITAI components in the R33 phase. Following initial development of the new JITAI, we will conduct a pilot and qualitative interviews with 10 low-SES NHPIF young adults engaging in co-use to assess feasibility. After additional intervention refinements, we will conduct an RCT (2-group) of the JITAI with low-SES NHPIF young adults recruited from community sources who co-use e-cigs and cannabis assigned to intervention (n=60) or ecological momentary assessment-only (n=60) to examine its impact on frequency and problem e- cigarette and/or cannabis use.