Project Abstract Despite the effectiveness of antiretroviral medications for HIV suppression, people with HIV who also have mental health and or substance use disorders suffer disparities in rates of HIV suppression and retention in HIV care. Evidence based treatment for mental health disorders (MHD) and substance use disorders (SUD), including use of medications and self-management support have been associated with improved engagement in HIV care. However, a key gap in many HIV care settings is an inability to consistently screen and provide treatment for MHD/SUD. Barriers include limited provider knowledge of, and comfort with, MHD/SUD screening and treatment; time constraints; and absence of support staff to facilitate linkage and retention in MHD/SUD care. We propose a Hybrid Type 1 implementation- effectiveness trial of an adapted collaborative care model with peer-specialists as care managers, to enhance screening and treatment for MHD/SUD with resultant improvement in rates of HIV viral suppression and care engagement in an HIV clinic setting. Collaborative care (CC), the use of a care manager/team to facilitate and coordinate integration of mental health and substance use services with primary care, is a well-established evidence-based model. We intend to adapt CC by training and supporting peer specialist to function as care managers. We will then rigorously evaluate peer-supported CC as a component of a multidisciplinary team including the HIV care provider and an addiction psychiatry consultant as an implementation strategy for MHD/SUD screening and treatment in a continuity HIV care practice. We will compare this peer-supported CC to usual care (UC) referral to psychiatric care or a SUD treatment program at the discretion of the primary care provider and evaluate the implementation process using the RE-AIM framework. Our specific aims are as follows Specific Aim 1: In a randomized trial, compare the proportion of patients with MHD/SUD with HIV suppression randomized to the collaborative care (CC) and the usual care (UC) referral strategy. We will randomize 400 patients 1:1 to CC versus UC and compare the proportion of patients with HIV suppression (defined as HIV RNA <200 copies/ml) at 12 months. Specific Aim 2: Characterize processes of CC implementation including reach, adoption, fidelity, and provider acceptability, maintenance over time, and costs associated with the approach. We propose a mixed methods approach to measure the implementation processes guided by the RE-AIM Framework. In sub Aim 2b, we will conduct a cost analysis for the peer-supported CC model. Specific Aim 3: Determine the mediators of the CC intervention on HIV viral load suppression and retention in care. We will utilize a structured questionnaire and clinical data to assess mediators of the CC intervention on HIV outcomes and qualitatively assess overall experiences with the peer supported CC model and explore if patient characteristics impact experience.