PROJECT SUMMARY There is a growing mental health crisis in the US, with the prevalence of clinically significant psychological distress growing while access to behavioral health services apparently worsening. There has been a particular shortage in the availability of crisis services and inpatient beds, placing a greater burden on emergency departments and potentially associated with higher rates of homelessness, incarceration of persons with serious mental illness, and worse health and wellbeing. The shortage of inpatient psychiatric beds can be tied in part to Medicaid policies that do not allow federal payment for stays in “Institutions for Mental Diseases” (IMDs), facilities with more than 16 beds that specialize in behavioral health treatment. Recently, the Centers for Medicare & Medicaid Services (CMS) created an opportunity for states to apply for waivers to receive federal payments for stays in IMDs for adults with serious mental illness (SMI) and children with serious emotional disturbances (SED). Eleven states have received an SMI/SED waiver, and five are pending. The waivers have the potential to improve crisis care and outpatient mental health care, potentially reducing emergency department use and boarding, improving the coordination of care, and reducing suicide and suicide-related behaviors. However, it is not yet clear whether the waivers have led to changes in services or what state implementations are most successful in improving outcomes. This comprehensive mixed-methods study will assess the effects of removing the IMD exclusion through the SMI waivers. The staggered waiver approval process creates a natural experiment to compare states that implement the waiver to those that do not. Our quantitative analysis will use the Transformed Medicaid Statistical Information System Analytic Files (TAF), a national Medicaid data set that includes information on eligibility, enrollment, and claims related to service utilization and prescription use. Our qualitative analysis will explore state-level factors that may contribute to changes in outcomes. In response to PAR-21-316, our proposed study will identify policies and factors that affect access, utilization, and outcomes, using large Medicaid claims datasets and leveraging an existing data management approach to advance our research.