Most mental health problems onset between the ages of 10-24. At the same time, adolescence is a period of life characterised by an increased susceptibility to peer influence effects. Despite this, we have limited knowledge regarding the individual differences that increase susceptibility to peer influence effects on adolescent mental health. Adolescents spend a considerable amount of time at school, among peers, and knowing who is susceptible to peer influence effects on mental health will help identify those at risk, and highlight potential intervention targets. Indeed, peer influence effects on mental health might also underpin the success or failure of universal school- based interventions for mental health, which are delivered to adolescents in peer groups. Yet to date, peer influence effects have not been considered when examining the effectiveness of these interventions. This may explain why we see heterogenous outcomes, with some interventions succeeding, and others failing. This proposal, therefore, aims to reduce the discontent between our understanding of peer influence effects and adolescent mental health by (1) isolating the individual differences that increase susceptibility to peer influence effects on mental health problems, and by (2) examining whether these effects explain the success or failure of universal school-based interventions for mental health.