Objective: Although current UK policy argues that schools have a key role in raising health standards, emphasis on the core curriculum restricts teachers' opportunities to undertake health promotion activities. The challenge is to design effective health promotion interventions that minimize pressures on teaching staff and curriculum space. Here we consider teachers' perspectives of an effective peer-led, school-based smoking intervention, implemented by external trainers. Design: The intervention, during which influential Year 8 students identified through a whole-year peer nomination process were trained to reduce smoking uptake through informal interactions with students in their year group, was evaluated by a pragmatic randomized controlled trial (ASSIST: A Stop Smoking In Schools Trial). An integral process evaluation examined the context, implementation and receipt of the intervention. Setting: Thirty secondary schools in south-east Wales and the west of England. Methods: Teachers in all intervention schools completed questionnaires at key stages of the intervention. Semi-structured interviews were undertaken in four schools at baseline and immediately post-intervention. The method of constant comparison, derived from grounded theory, was used throughout the analysis. Results: The intervention was successfully implemented in a wide range of schools; recruitment and retention rates were good; and outcome data showed a reduction in smoking levels. Some teachers expressed concern about the participation of challenging students, external trainers setting standards of discipline, and communication over timetabling. Conclusion: Overall, teachers showed commitment to the ASSIST intervention and felt it was compatible with the Year 8 curriculum. If implemented more widely, the importance of peer nomination should be stressed. (Contains 5 tables and 2 figures.)