SUMMARY/ABSTRACT Arguably the most important question within the field of addiction treatment today is how to close the implementation gap between evidence-based treatment and the fraction of people who receive care; medication for opioid use disorder (MOUD) is a case in point. Efforts to close this gap require accurate data to monitor performance and direct quality improvement efforts across the different systems in which patients receive care, and for different subpopulations. The cascade of care framework, originally developed for HIV, is a conceptual model that has been applied to OUD to assess and improve system performance; to date it has only been applied to single systems of care for OUD. However, OUD care is fundamentally different from HIV care and MOUD delivery has evolved since the cascade framework was proposed. Policy changes, such as the ability to provide MOUD through telehealth and the removal of the X-waiver, have changed how patients receive care. Thus, the current OUD cascade of care model may be inaccurate and insufficient for quality improvement efforts. Rather, a cross-system approach is needed that specifically accounts for different care trajectories utilized by patients with OUD. For example, MOUD can be initiated prior to linkage, (e.g., through emergency department or inpatient settings) and patients may move between primary and specialty care, necessitating the ability to link data from different systems. This project proposes modifying the OUD cascade of care model to reflect the changing landscape of real-world OUD treatment, enabling us to answer critical research questions about treatment gaps for patients identified with OUD who receive care across different public-sector systems. In doing so, we advance the science of performance measurement for OUD and identify opportunities for cross-system quality improvement efforts. We also examine differences in care by population and facility characteristics—essential information for organizing and prioritizing intervention efforts to have the biggest impact on limiting the opioid crisis. Although we use OUD as an exemplar condition, our long-term goal is to describe the care cascade for patients with other substance use disorders (SUD) beyond OUD. We conduct our study in Los Angeles County (LAC), which had a nearly 50% increase in overdose deaths from 2019-2020; these deaths predominantly involved fentanyl and disproportionately were from poorer areas and the safety net population, who have limited access to care. Our proposal builds upon an existing partnership between RAND, the Department of Health Services (the second largest county health system in the US by patient volume; includes primary care, emergency department and inpatient care) and the Department of Public Health Substance Abuse Prevention and Control Program (oversees and funds all specialty public SUD treatment in LAC, including methadone). This will be the first study to link data from both the healthcare...