PROJECT SUMMARY/ABSTRACT To reach the Ending the HIV Epidemic (EHE) targets for reducing new HIV infections by 2030, King County WA must improve its plan to reach individuals with behavioral health disorders. County surveillance data reveal very high rates of mental disorders and substance use disorder among new HIV infections since 2018. Research has demonstrated that the HIV prevalence among people with serious mental illnesses (such as schizophrenia and bipolar disorder) is at least four times that of the general population, but that these individuals are inconsistently tested and often missed through HIV screening efforts. Most people with the US who have serious mental illness access the healthcare system through their mental health provider, but rates of screening and prevention services are low in community mental health settings. The goal of this research is to increase the capacity of King County behavioral health organizations to provide rapid HIV testing and linkage to proven prevention services (pre-exposure prophylaxis and syringe services programs that are the focus of the Prevent Pillar of the King County EHE plan) to reduce HIV incidence in King County. Current EHE efforts include Learning Collaboratives--a widely-used implementation strategy to support implementation of new services or improve care—to increase HIV testing and linkage to services in emergency departments and primary care clinics. The current one-year proposal aims to develop an EHE Collaborative to support behavioral health organizations to increase access to HIV testing and linkage to services among a critical population that is not being reached by current EHE efforts. We propose to identify best practices for HIV prevention services and barriers and facilitators to implementing a status neutral approach to HIV services. We will utilize a mixed methods approach, collecting both quantitative data from an inventory of services and resources completed by directors of the 25 member organizations of the King County Behavioral Health Provider Association and qualitative data through in-depth interviews of 45 individuals who represent key stakeholder groups in efforts to improve HIV prevention services in behavioral health organizations. This proposal leverages a unique opportunity to layer rigorous implementation science research on existing EHE activities, to generate knowledge both about how to build HIV prevention capacity in behavioral health organizations and how to evaluate EHE Learning Collaboratives. At the end of the proposal, we will have a tailored EHE Collaborative for Behavioral Health organizations which we will test in a subsequent implementation trial.