Drug use is a cause of health and social problems in the UK. Despite widespread interest in whole-school interventions, which make changes to school socio-environmental factors in order to reduce young people's drug use, the evidence-base regarding 'school effects' on drug use had not previously been synthesised, and the basis for these potential effects only partly theorised. This thesis examines whether and how schools as institutions can influence young people's attitudes and actions relating to drug use and generate new theoretical insights regarding the processes through which any school effects on students' drug use may occur.