ABSTRACT The US is currently on pace to fall short of achieving the goals set by the Ending the HIV Epidemic (EHE) initiative launched by the Department of Health and Human Services in 2019. A growing body of research suggests that the root causes of EHE's striking lack of progress are structural racism and social stratification that leads to poverty. Housing instability — a lack of stable, secure, and adequate housing that results from poverty and structural racism — is a major social determinant of health (SDOH). The Philadelphia Department of Health (PDPH) estimates that 4,000 people with HIV (PWH) experience housing instability in Philadelphia. While the National HIV/AIDS strategy specifically calls for approaches to address SDOH (including through the development and scaling up of housing interventions), the vast majority of EHE efforts continue to focus on facilitating healthcare access and uptake. There have been few rigorous evaluations of housing interventions for PWH, and even fewer during the modern era of universal treatment for HIV. In close partnership with the PDPH, we will rigorously evaluate a potentially transformative city-level transitional housing program for PWH experiencing housing instability (“Arms Around You” or AAY), leveraging lottery drawings from a waitlist to estimate causal effects. The program will be implemented by PDPH in 2024 and scaled up in the coming years (~200 anticipated enrollments during the first several years). AAY includes (1) intensive housing counseling, (2) housing medical case management, and (3) full rent payment support for 48 months (or longer if needed). We will use mixed methods to collect quantitative (survey data, pharmacy refill data, and PDPH surveillance/program data) and qualitative (key stakeholder interviews) data. If the AAY screening period begins before this proposal's full review by NIH, we have pilot funding from PDPH to collect baseline data. Since PDPH's lottery drawing is necessitated by high anticipated demand (PDPH estimates there are 4,000 PWH in Philadelphia experiencing housing instability) and limited initial availability (~200 available spots during the first few years), we will be able to estimate the long-term effects of the program on health, economic, and psychological outcomes. In addition to studying mechanisms through which housing interventions affect health outcomes, we will use implementation science methods to assess program acceptability, reach, sustainment, and costs to inform scalability to other cities. This proposal is innovative in that we will capitalize on a unique opportunity to use rigorous causal and implementation science methodology to evaluate the health and non-health effects of a population-level large- scale housing intervention that addresses an important SDOH. This approach could significantly impact both HIV- and non-HIV-related outcomes, such as viral suppression, ART adherence, engagement in care, substance use, financial well-being, ho...