Project description: Few evidence-based interventions (EBIs) have been adapted for youth in foster care to specifically address substance use prevention during early adolescence. Even when youth in foster care are referred to EBIs, prior work demonstrates they experience difficulty accessing services. This R01 grant project leverages a robust existing service delivery program for youth in foster care, Court Appointed Special Advocates (CASAs), to tailor and deliver an EBI specifically focused on trauma-informed substance use prevention. Working with CASAs, the research team will use a hybrid type 1 effectiveness implementation randomized trial to test whether the adapted intervention improves substance use and health outcomes for foster youth compared to CASA services-as-usual. The project will test the intervention in the setting in which it is intended to be delivered to decrease the research-to-practice gap for youth involved with child welfare. If positive preventive effects are found, the potential for scalability and reach for youth in foster care is exceptional, given that CASA is currently in 49 U.S. states, serving approximately 242,000 (60%) youth in foster care.