ABSTRACT People with opioid use disorder are often forced to undergo withdrawal during incarceration due to lack of opioid treatment in custody, contributing to their elevated risk of drug overdose and other negative health outcomes in the weeks after release. Providing access to medications for opioid use disorder (MOUD) in correctional settings is a promising strategy to reduce overdose risk, but jails have not widely adopted this strategy. Achieving maximal public health benefits will require that MOUD programs are adopted across local jails that vary substantially in their resources and capacity, that benefits diffuse widely across racial/ethnic groups, and that jails ensure transition to treatment providers in the surrounding community after release. In 2019, the Maryland legislature passed the first U.S. law requiring all local jails in the state to provide all forms of MOUD and create linkages to care after release. The legislation created a phased implementation of the program starting in 2020. The staggered implementation presents a natural experiment to evaluate how the implementation of the programs in jails affect a variety of post-incarceration outcomes overall, and across different facilities and local environments. We propose a mixed-methods evaluation of the Maryland law. Aims 1 and 2 will analyze a data warehouse that links statewide correctional records, hospital, prescription monitoring program, behavioral health, and medical examiner data. Aim 1 will consider overall impacts of exposure to the jail MOUD program using non-experimental methods that compare changes in post-release outcome before and after jails implement the program. Aim 1B will use implementation measures reported to the state to assess whether outcomes differ in jails with high versus low levels of implementation based on monthly reports of program participation. Aim 2 will examine racial/ethnic disparities in the impact of the policy, and assess the contribution of variables at the individual, jail, and community level (e.g., social stressors, access to treatment) on inequitable outcomes. We hypothesize that overdose and other hospitalizations will decrease and MOUD use will improve among individuals leaving facilities with MOUD programs. We also hypothesize that impacts will be strongest for people treated at programs with high (versus low) levels of implementation. We hypothesize that people from minoritized backgrounds will not gain as much from the policy as non-Hispanic white people due to pervasive inequities in access to resources in jails and post- release. Aim 3 will provide complementary implementation evidence focusing on 8 jails. We will conduct in- depth, semi-structured interviews with 40 program leaders (e.g., jail staff, community providers) and 40 formerly incarcerated individuals to explore barriers and facilitators to providing MOUD in jail and post-release. Together, study Aims will provide new evidence about how statewide jail initiatives ...