The purpose of this study is to evaluate the implementation and effectiveness of a flexible, multidisciplinary, integrated drop-in/mobile HIV care approach for people living with HIV (PLH) who are not well engaged in current care systems (i.e. scheduled HIV primary care visits). The hybrid type 2 implementation-effectiveness study involves a set of implementation strategies to support implementation of the integrated drop-in/mobile HIV care approach (i.e. the evidence-informed clinical intervention) at four diverse sites in San Francisco and Alameda counties in California. Sites include an academic clinic located at a public hospital (Ward 86) and a needle exchange site (San Francisco AIDS Foundation Syringe Access Site) in San Francisco and two Federally Qualified Health Centers serving diverse patient populations in Alameda County (Trust and La Clínica). The evidence-informed clinical intervention consists of four key components: 1) active referral to care sites; 2) drop-in, multidisciplinary HIV primary care; 3) mobile HIV care; and 4) staged escalation/de-escalation of care level as needed. The study will use RE-AIM to guide evaluation, with coprimary outcomes of Reach and HIV viral suppression, and mixed methods to assess intervention Adoption, Implementation, and Maintenance. The study draws on the CFIR framework to assess site-specific implementation determinants before and after the study period. The study will undertake micro-costing using a uniform cost data collection protocol to quantify the resources needed to carry out intervention activities.