PROJECT SUMMARY Despite growing overdose fatalities primarily driven by opioids, only 10% of people with a substance use disorder (SUD) receive treatment. Yet evidence-based medication therapies that can reduce overdose risk exist, such as long-term buprenorphine maintenance therapy, but are vastly underutilized. As the most common insurer for people with opioid use disorder (OUD), the Medicaid program is uniquely positioned to improve access to evidence-based care and curb the tremendous toll of the overdose epidemic. However, prior work shows Medicaid beneficiaries face steep barriers accessing MOUD in outpatient settings due to low Medicaid provider participation and half of providers not accepting new Medicaid patients. Residential treatment programs are even less likely to participate in Medicaid than the outpatient setting due in part to the Institution for Mental Diseases (IMD) exclusion, a long-standing policy prohibiting federal matching of Medicaid payments for residential addiction treatment. Starting in 2015, states were able to apply for 1115 Substance Use Disorder IMD waiver demonstration projects, which would permit federal reimbursement for residential treatment. Waiver approval and renewal was contingent upon states proposing a plan to improve the accessibility and quality of their addiction treatment systems at large for Medicaid beneficiaries, including improving access to life-saving therapies such as buprenorphine. The long-term goal is to facilitate improved outcomes for individuals with SUD through transformation of the addiction treatment industry. The objective of the current application, which is the next step towards that goal, is to evaluate if IMD SUD 1115 waiver implementation is associated with changes in whether residential and outpatient treatment programs improve access to evidence-based behavioral health therapies for medically under-served populations. I use a quasi-experimental approach and consider outcomes in IMD waiver states before and after waiver implementation relative to states without waivers. The project’s PI and dissertation committee is an interdisciplinary team of health services researchers, health economists, and a physician with the methodological and substantive expertise to examine this understudied issue and explore its complex public health, clinical, and policy implications. The specific aims are to assess whether IMD waiver implementation is associated with changes in whether facilities accept Medicaid, offer any buprenorphine therapy (verses none), and offer buprenorphine maintenance therapy (verses no maintenance). The project is significant, as it focuses on how to structure reforms to address a key challenge in addiction treatment: incentivizing treatment programs to participate in Medicaid and offer evidence-based care. It is timely, as most waivers are coming up for review and findings can inform the Centers for Medicare and Medicaid Services’ renewal review process.