PROJECT SUMMARY / ABSTRACT In response to PAR-20-036: Getting to Zero: Understanding HIV Viral Suppression and Transmission in the United States, we propose a hybrid type 2 effectiveness-implementation study that will evaluate an innovative clinic-level intervention featuring an evidence-based mobile health (mHealth) application (CHESS) and peer- driven social support. Within the infrastructure of a large HIV/AIDS Service Organization offering an integrated, patient-centered model of care in Colorado, Missouri, Texas and Wisconsin, we will implement an mHealth system designed to close information gaps, build intrinsic motivation, and develop behavioral skills needed for sustained adherence to treatment. Peer support will be provided through activities delivered by patients recruited and trained to serve as peer mentors. The mHealth system and peer mentoring will be integrated into the existing care model, known as the HIV Medical Home. We hypothesize that the integrated intervention will increase the proportion of patients with viral suppression and reduce missed clinic appointments by supporting three needs: (1) It will facilitate real-time, community-based capture of data reflecting social and behavioral determinants known to precede lapses in HIV care (e.g., housing and food insecurity, unhealthy alcohol or drug use, or poor medication adherence); (2) It will improve engagement in care by increasing social connectedness among patients, and between patients and peer mentors; and (3) It will support retention in addiction treatment and mental health care that help maintain engagement in HIV care. Project Year 1 will be devoted to planning and refinement of the intervention in close collaboration with organizational leaders and people living with HIV who are members of the local communities. Beginning in Project Year 2, we will conduct a stepped wedge cluster randomized trial of the intervention in seven clinics. We will leverage data collected through the integrated electronic health record system serving all seven clinics to test intervention effectiveness on viral suppression and retention in care. We also will conduct an implementation cost analysis and cost-effectiveness analysis to inform future sustainment of the intervention model. The study builds on a solid foundation of prior research demonstrating the promise of mHealth and peer strategies for enhancing engagement in HIV and addiction treatment and care for other complex conditions. The findings from this study, if successful, will contribute a new and innovative set of tools with high potential impact for improving HIV viral suppression in multiple geographic settings.