Background: Despite a decline in child and adult smoking prevalence, young people who smoke (even occasionally) can rapidly become addicted to nicotine, with most adult smokers initiating smoking before they are 18. Schools have long been a popular setting to deliver youth smoking prevention interventions, but evidence of the effectiveness of school-based prevention programmes is mixed, and outcomes vary by the type of programme delivered. Existing systematic reviews that explore the factors contributing to the success or failure of school-based smoking prevention programmes often exclude qualitative studies, due to a focus on intervention effectiveness which qualitative research cannot answer. Instead, qualitative research is focussed on t...