INTRODUCTION: Peer education interventions are widely used in secondary schools with an aim to improve students' health literacy and/or health behaviours. Although peer education is a popular intervention technique with some evidence of effectiveness, we know relatively little about the key components that lead to health improvements among young people, or components that may be less helpful. This review aims to identify the main mechanisms involved in school-based peer education health interventions for 11-18-year-olds. METHODS: Five electronic databases were searched for eligible studies during October 2020, an updated search was then conducted in January 2023 to incorporate any new studies published between November 2020 and January 2023. To be included in the review, studies must have evaluated a school-based peer education intervention designed to address aspects of the health of students aged 11-18 years old and contain data relevant to mechanisms of effect of these interventions. No restrictions were placed on publication date, or country but only manuscripts available in English language were included. RESULTS: Forty papers were identified for inclusion with a total of 116 references to intervention mechanisms which were subsequently grouped thematically into 10 key mechanisms. The four most common mechanisms discussed were: 1) Peerness; similar, relatable and credible 2) A balance between autonomy and support, 3) School values and broader change in school culture; and 4) Informal, innovative and personalised delivery methods. Mechanisms were identified in quantitative, qualitative and mixed methods intervention evaluations. DISCUSSION: This study highlights a number of key mechanisms that can be used to inform development of future school-based peer education health interventions to maximise effectiveness. Future studies should aim to create theories of change or logic models, and then test the key mechanisms, rather than relying on untested theoretical assumptions. Future work should also examine whether particular mechanisms may lead to harm, and also whether certain mechanisms are more or less important to address different health issues, or whether a set of generic mechanisms always need to be activated for success.