Project Summary/Abstract This mixed methods study aims to assess whether peer-support reduces disparities in mental health service utilization and social outcomes among transitional age youth (TAY) age 16-24 with severe mental illness (SMI). TAY with SMI face extraordinary challenges in their transition to adulthood and exhibit lower levels of educational attainment, and higher rates of unemployment, poverty, unplanned pregnancy, substance use, homelessness, and justice system involvement than TAY without SMI. These disparities are exacerbated among minority TAY. Therefore, there is a great need for evidence-based, culturally tailored interventions that effectively link TAY to resources that can improve their mental health and social outcomes. The 2004 California Mental Health Services Act funded hundreds of new TAY-specific programs; while TAY peer support services are now pervasive in mental health programs, evidence regarding their effectiveness for TAY with SMI is largely lacking. Therefore, this study aims to (1) Estimate quantitatively whether the use of peer support reduces racial / ethnic disparities in mental health services and social outcomes among TAY living with SMI; (2) Identify quantitatively which types or combinations of peer supports are most effective at improving mental health services and social outcomes and reducing racial / ethnic disparities among TAY; (3) Estimate the additional mental health service costs or cost savings associated with providing peer support services to TAY; (4) Qualitatively elucidate TAY clients’ and peer providers’ perceptions of how TAY peer support relates to mental health services and social outcomes disparities; and (5) Qualitatively characterize barriers and facilitators to the use of TAY peer support by TAY program managers and county administrators. To meet these aims, we will survey TAY programs (N=140), analyze five years of administrative data to assess costs or cost-savings associated with using peer support services (N=8,800, Aims 1-3) and conduct small group interviews with TAY peer providers (n=30-40), focus groups with TAY clients (n=60-80), and individual interviews with as TAY program managers (n=20) and county administrators (n=6-10) (Aims 4 and 5). This study leverages a major system redesign in California’s mental health system and aims to rigorously study a large-scale implementation of TAY peer support services. Findings will help providers and policy-makers make informed decisions about the effectiveness of peer support in reducing disparities among minority TAY living with SMI and enable us to make recommendations on potential best-practices in the use of TAY peer supports.