This article explores factors that may influence hand washing behaviour among pupils and staff in primary schools. A qualitative process evaluation within a cluster randomized controlled trial included pupil focus groups (n = 16, aged 6-11 years), semi-structured interviews (n = 16 teachers) and observations of hand washing facilities (n = 57). Pupils and staff in intervention and control schools demonstrated a similar level of understanding of how, when and why they should wash their hands. Lack of time, poor adult modelling of regular hand washing and unattractive facilities were seen as important barriers to regular hand washing. Reminders and explanations for the importance of hand hygiene were thought to have a positive impact. Influencing individual choices about hand washing through education and information may be necessary, but not sufficient, for initiating and maintaining good hand washing practices. Structural factors, including having time to wash hands using accessible, clean facilities, and being encouraged through the existence of hand washing opportunities in the daily routine and hand washing being viewed as the social norm, will also influence hand washing behaviour. The effectiveness of educational interventions at improving hand hygiene in primary schools may be improved by changing priorities of staff and increasing accessibility to quality facilities.