Peer bullying increases in times of school transition, influenced by changing peer and friendship groups, new schooling environments and greater stress. Covert forms of bullying, including cyberbullying, become more common in secondary school and cause considerable distress and longterm harm. The period of transition to secondary school is therefore a critical window for intervening to manage and prevent bullying. A three-year cluster randomised control trial was conducted to develop, implement and evaluate the Friendly Schools Project intervention which aimed to reduce bullying and aggression among more than 3,000 students who had recently transitioned to secondary school.Intervention schools were provided with individualised training and resources to support students' transition and reduce bullying using a multi-level comprehensive intervention addressing classroom curriculum, school policies and procedures, the social and physical environment, pastoral care approaches and school-home-community links.Although the observed effect sizes were small, the intervention had a consistently significant positive effect across a range of outcomes, including bullying perpetration, 496 | CROSS et al.