Abstract Medication assisted treatment (MAT) is an evidence-based treatment for opioid use disorder (OUD) involving medication and behavioral therapy. Despite massive public expenditures to increase MAT access, MAT use remains low. Limited geographic access to MAT is a key barrier to OUD treatment because MAT requires frequent travel to providers. While expanding geographic access to MAT has been a policy priority, policymakers lack reliable, accurate measures of geographic access to MAT to guide planning efforts. The overall objectives of this proposal are to determine where and for whom geographic access to MAT is limited, and to examine how geographic access to MAT affects OUD-related health outcomes. The hypotheses are that geographic access to MAT is lower for disadvantaged socioeconomic groups, and that increased geographic access to MAT improves OUD-related health outcomes. The rationale for this project is that improved measurement of geographic access to MAT will enable policymakers to implement targeted, effective interventions to increase MAT access and use. The applicant will use Pennsylvania Medicaid claims to address the following specific aims: (1) Characterize variation in geographic access to MAT providers by car and public transit; (2) Identify patient and environmental characteristics associated with geographic access to MAT; and (3) Determine the impact of geographic access to MAT on MAT use and OUD-related overdoses. This application is innovative because it will apply novel methods for measuring geographic access to MAT, including disparities in access by public transit (which is particularly relevant to low-income populations in Medicaid), and use these measures with causal inference methods to estimate the causal effect of geographic access to MAT on OUD-related outcomes (including receipt of MAT and opioid overdoses). The proposed research is significant because it will enable policymakers and researchers to accurately identify small geographic areas where targeted interventions to improve geographic access to MAT are needed. It also will estimate how effective such interventions would be. Ultimately, the proposed research will create an evidence basis from which policymakers can prioritize interventions and allocate resources to efficiently increase geographic access to MAT. Through a training plan developed with the support of a multi-disciplinary mentorship team of NIH-funded investigators, this project will enable the applicant to develop a working knowledge of the treatment of OUD and expertise in MAT policy and geospatial methods. This project also places an emphasis on dissemination of results to state Medicaid administrators. Collectively, this project will prepare the applicant to transition to independence by submitting a R01 application to NIDA examining how various state policies affect OUD-related outcomes through their effects on geographic access to MAT.