Objective: To design, implement and evaluate a peer-led intervention to reduce smoking amongst secondary school students. Design: A health promotion intervention combining peer education with diffusion of innovation theory, to be rigorously evaluated by means of a cluster randomised controlled trial with concurrent process and economic evaluations. Setting: Year 8 students (12/13 year olds) in 30 secondary schools in south-west England and south Wales. Method: Approximately 15 per cent of students, identified by their peers as being influential within the school, were trained to intervene in everyday situations and encourage their fellow students not to smoke. These "peer supporters" received two days of intensive training from a team of trainers led by professional health educators at training venues and four follow-up sessions back at school. The training aimed to equip the peer supporters with the knowledge, skills and confidence to encourage their peers not to smoke. Results: The intervention proved feasible and acceptable to schools. 10,731 Year 8 students were recruited to the trial at baseline. Retention of peer supporters throughout the ten-week intervention period was high. Eighty- two per cent (687 of 835) of students who consented to act as peer supporters completed the programme and fulfilled the role. The average cost per school of delivering the intervention was 4,702 British Pounds. Conclusion: The costs of implementing this programme were sizeable but, if effective, it could yield substantial long-term health gains and contribute to a reduction in health inequalities. (Contains 4 figures and 2 tables.)