PROJECT SUMMARY The United States (US) is in the midst of an epidemic: opioid use disorder?and associated, overdose deaths?have risen at an alarming rate in recent years. In Rhode Island (RI) and several other states, the epidemic of opioid overdose has been exacerbated by contaminated heroin supplies. In 2015 and 2016, fentanyl and fentanyl analogs with higher potencies than heroin have been involved in increasing numbers of overdose deaths. For those who have been recently incarcerated, rates of opioid use disorder and overdose are amplified. In addition, those with criminal justice (CJ) involvement have experienced an exaggerated increase in fentanyl-related opioid overdoses. In RI in 2015, 70% of those with criminal justice (CJ)- involvement in the two years before death had a fentanyl-related overdose, an increase from 38% in 2014. Therefore, there is a dire need to address the rising number of overdose deaths among those with recent CJ involvement. Medication assisted therapy (MAT) is the most effective evidenced-based approach to treat opioid use disorder. However, even though MAT has been proven effective, there has been little to no implementation and routinization of MAT in US CJ settings. Recently the governor of RI convened a strategic task force on overdose prevention and intervention. A major advancement of this task force was the recommendation and subsequent implementation of a comprehensive MAT program to be delivered at the unified jail and prison system managed by the RI Department of Corrections (RIDOC). Therefore, we are proposing to conduct an evaluation of the implementation and impact of this novel comprehensive MAT program which will include: 1) a mixed-method assessment of the implementation process of the comprehensive MAT program using the Evaluation, Preparation, Implementation and Sustainment (EPIS) framework; and 2) determining whether the implementation of the comprehensive MAT program reduces the rate of fatal opioid overdose among individuals with recent CJ-involvement in RI. The results from this R21 will provide guidance to other jurisdictions that are considering creation and implementation of scaled up MAT interventions. Additionally, we intend to develop an R01 application to launch comprehensive MAT interventions at correctional sites across the country. Through this research, we hope to fundamentally change the way in which treatment for opioid use disorder is provided to CJ-involved populations.