Engaging the most vulnerable people with HIV in treatment is central to efforts to end the HIV epidemic. Low- barrier care (LBC) is an evidence-based multicomponent intervention that includes walk-in access to medical care, incentives for clinic visits and viral suppression, intensive support to address social determinants of health, and multisectoral service coordination. The Max Clinic in Seattle is a flagship LBC clinic that has substantially improved viral suppression among people with complex barriers to care, including unstable housing, substance use, and mental health disorders. Expanding LBC is central to the Ending the HIV Epidemic (EHE) plan in King County, Washington. The overall goals of this 5-year proposal are to study LBC expansion in King County, identify factors that facilitate LBC implementation, and prepare for the intervention’s scale-up in other EHE jurisdictions. The team will employ a multifaceted implementation strategy to establish and improve two new LBC clinics in addition to the Max Clinic. We will use the Reach, Effectiveness, Adoption, Implementation & Maintenance (RE-AIM) framework and structured tools to assess intervention adaptation and scalability. For Aim 1, we will evaluate the reach and effectiveness of LBC expansion in King County using a population-based observational open cohort design. For Aim 2, we will describe the implementation and maintenance of LBC in King County, including intervention adaptations, organizational strategies to overcome barriers, and costs. Using the Framework for Reporting Adaptations and Modifications-Enhanced (FRAME), we will conduct focus group discussions with key stakeholders in the LBC clinics and the health department at baseline and conduct brief re- assessments in six-month intervals over four years to describe intervention adaptations and elucidate implementation and maintenance strategies. We will quantify the costs of LBC expansion from the health department and healthcare organization perspectives using micro-costing methods. For Aim 3, we will assess the scalability of the LBC intervention and identify a consortium of health department and clinic leaders to scale- up LBC in cities and states prioritized for the first phase of EHE funding throughout the U.S. We will partner with NASTAD (the National Association of State & Territorial AIDS Directors) to identify health department HIV program leaders supportive of LBC implementation, and in collaboration with them, recruit leaders of Ryan White-funded HIV Clinics or Community Health Center clinics. We will conduct in in-depth interviews with both the health department and clinic leaders using the Intervention Scalability Assessment Tool (ISAT) for applied assessment of LBC implementation considerations in each local context. Expected Outcome: Our results will guide optimization of LBC implementation, inform intervention adaptations, and lay the groundwork for strategic scale-up of LBC in other parts of the country, which...