Background: The sustainability of school-based health interventions to improve students’ health, wellbeing and behaviour after start-up funding/resources cease has been relatively unexplored compared to health-care. Discontinuing effective interventions prevents new practices from reaching wider student populations and wastes investment in implementation. This thesis examines evidence on whether/how schools sustain health interventions and explores sustainability processes. Method: Empirical studies were systematically reviewed to assess the sustainability of school health interventions. A case-study was conducted of the sustainability of ‘Learning Together’, a bullying-prevention intervention initiated in English secondary schools through an effectiveness trial. The intervention entailed: restorative practice (RP), a staff-student action group, and a curriculum. Qualitative, longitudinal data were collected from five schools: interviews with multiple staff/school, and with students and external facilitators the first-year post-trial; interviews with one staff member/school two years post-trial; and descriptive data from the trial’s process evaluation. Results: Twenty-four studies of eighteen interventions were included in the systematic review. No interventions were sustained entirely; all interventions had some components sustained by some schools/staff, bar one that was discontinued. Key facilitators included commitment from senior leaders, staff observing an improvement in students’ engagement/wellbeing, and confidently delivering valued intervention approaches. Important contextual barriers emerged: the norm of prioritising educational outcomes under time/resource constraints, insufficient funding/resources and ongoing training, and staff turnover. Learning Together was not sustainable two years post-trial. RP had been continued by some individuals in all schools and was sustained at school-level in one school; the curriculum and action groups were discontinued in all schools, though actions initiated by the groups were sustained in two schools. Staff’s experiences of components’ effectiveness compared to existing provision and views of their long-term value affected components’ sustainability. Sustainability depended on staff’s ability to mainstream desired components across the school, which they had little capacity to do. Conclusion: Intervention developers need to support schools to mainstream evidence-based interventions to sustain them at school-level. Methodologically stronger primary research on sustainability and sustainability strategies is needed.