PROJECT SUMMARY/ABSTRACT Amidst the current opioid overdose crisis, 1.4 million women of reproductive age report non-medical opioid use, and opioid use disorder (OUD) prevalence among pregnant women quadrupled from 1998-2014. Although pregnancy may be a motivating time to start medications to treat OUD, retention post-partum declines significantly and is associated with unacceptably high opioid overdose and mortality rates. Injection drug use is also the primary risk factor for hepatitis C virus (HCV) infection in the United States (U.S.). Women now comprise nearly half of new HCV infections, and HCV seroprevalence has risen five-fold in pregnant women since 2000. Although the U.S. is committed to eliminating HCV, about 50% of people are unaware of their infection and fewer than 40% of those diagnosed have been treated. Research is needed to identify venues to increase efforts to improve OUD treatment and retention and to identify and treat current HCV infection. Perinatal care provides longitudinal access to women over a nearly one-year period, and as pregnancy may be a motivating time for women to seek care for both OUD and HCV, perinatal care may be a critical venue to reach women who inject drugs, a key population that NIDA identifies as central to the ongoing opioid crisis. Yet, little data exist to measure the role of the perinatal care venue to improve OUD and HCV care engagement. This study aims to create a perinatal-specific decision model and measure health service utilization and costs of a perinatal OUD and HCV intervention to identify how to leverage the perinatal care venue to decrease postpartum opioid overdoses and population-level HCV transmission. This study has two main aims: (1) to build a microsimulation model to analyze the clinical and health economic effects of leveraging the perinatal care venue to reduce opioid overdoses and population-level and infant HCV transmission; (2) to measure health services utilization and costs of incorporating MOUD and HCV treatment into perinatal care. The results of this study could help determine most efficient resource allocation to achieve national overdose and HCV elimination goals. The PI, Dr. Epstein, is an early career investigator, trained in pediatric and adult infectious diseases, with a focus on addiction medicine and HCV in pregnant women and youth. This career development award will allow her to gain expertise in 1) transmission modeling and model calibration and 2) health economic evaluation and micro-costing to be able to conduct comparative cost- effectiveness research to inform interventions for women with substance use and HCV. She has compiled an experienced team of mentors and advisors to help her achieve her training and career goals, to become an independent clinician-investigator expert in utilizing decision science to project most effective use of resources to improve care for families at the center of the opioid overdose, HCV and HIV epidemics.