ABSTRACT Carrie E. Fry PhD, MEd is an Assistant Professor in the Department of Health Policy in the Vanderbilt School of Medicine. She is a health services and policy researcher whose portfolio is focused on improving health and social outcomes for Americans with a substance use disorder (SUD) or mental illness through rigorous observational and quasi-experimental methods. This training grant will support Dr. Fry’s career goal of becoming an independent researcher with expertise in evaluation strategies to inform policy affecting marginalized Americans. This type of expertise is in high demand: the incidence and prevalence of opioid use disorder (OUD) has significantly increased since the mid-2010s with related harms, including emergency room visits and fatal overdoses, increasing at a similar pace. Access to evidence-based OUD treatments, especially among Medicaid enrollees, has not increased at the same rate. For these reasons, understanding the Medicaid policy levers to increase access to treatment is crucial to combatting the broader SUD crises in the US. One way state Medicaid programs have responded is by expanding the continuum of care to include residential treatment. Historically, Medicaid programs not been allowed to provide residential SUD treatment for most adults. Following guidance issued in 2015, states could apply for time-limited, demonstration waivers to expand coverage for residential treatment to adults between 21-64 years old. Since then, 32 states have received approval for and implemented a Section 1115 waiver to provide residential SUD treatment. States have flexibility regarding the specific provisions of these waivers potentially leading to mixed outcomes, and the impact of expanded residential treatment coverage in Medicaid is largely unknown. The research proposed in this K01 will address these gaps to inform future policy. Aim 1 will survey Medicaid officials in the 13 states participating in the Medicaid Distributed Research Network (MODRN) to identify variation in Medicaid residential treatment coverage. Then, I will conduct semi-structured interviews with Medicaid and other state officials from six MODRN states to further explore these differences. Aim 2 will use Medicaid claims data from three states to produce reliable and generalizable evidence on the effect of expanded coverage for residential treatment on beneficiary-level health and utilization outcomes. Details gleaned from these survey and interviews will be used to appropriately contextualize Aim 2’s results; the evaluation proposed in Aim 2 can be completed regardless of these findings. Finally, Aim 3 will produce the first generalizable estimates of co-morbid SUD among Medicaid enrollees with OUD, a subpopulation more likely to access residential treatment. Results from these aims will provide extensive preliminary data for an R01 evaluating these coverage expansions among sub-populations identified in Aim 3 and inform future Medicaid policy around resid...