<p>Serious workforce shortages in youth mental health widen the gap between demand and use, and exacerbate health disparities. A strategy to address this is training and integrating youth and young adult (Y/YA) peers to deliver services. This paper reviews international scientific studies on Y/YA services targeting mental health conditions or health promotion. We applied systematic criteria including controlled designs (RCTs or quasi-experimental), active intervention delivered by a young person, key search terms, and peer-reviewed publication between 2012 and 2023. The review yielded 32 studies meeting criteria, encompassing six domains: eating disorders, health promotion, depression/anxiety, suicidal risk, serious mental illness (SMI) and “other.” The most consistent evidence for effectiveness was for eating disorders; findings in the other domains were mixed. Training manuals, fidelity measurement, reporting of demographic data, and consistency in outcome measurement were largely absent, limiting both replicability and generalizability. The acceleration of scientific attention to youth/young adult peer services over the past decade and emerging support for its effectiveness is encouraging, however, and it bodes well for mitigating workforce shortages and, more importantly, for improving the quality of services for young people.</p>