The aim of the study was to explore what components of the General Medical Council's (GMC) Quality Assurance Framework work, for whom, in what circumstances and how?UK undergraduate and postgraduate medical education and training.We conducted interviews with a stratified sample of 36 individuals. This included those who had direct experiences, as well as those with external insights, representing local, national and international organisations within and outside medicine.The GMC quality assure education to protect patient and public safety utilising complex intervention components including meeting standards, institutional visits and monitoring performance. However, the context in which these are implemented matters. We undertook an innovative realist evaluation to test an initial programme theory. Data were analysed using framework analysis.Across components of the intervention, we identified key mechanisms, including transparent reporting to promote quality improvement; dialogical feedback; partnership working facilitating interactions between regulators and providers, and role clarity in conducting proportionate interventions appropriate to risk. The GMC's framework was commended for being comprehensive and enabling a broad understanding of an organisation's performance. Unintended consequences included confusion over roles and boundaries in different contexts which often undermined effectiveness.This realist evaluation substantiates the literature and reveals deeper understandings about quality assuring medical education. While standardised approaches are implemented, interventions need to be contextually proportionate. Routine communication is beneficial to verify data, share concerns and check risk; however, ongoing partnership working can foster assurance. The study provides a modified programme theory to explicate how education providers and regulators can work more effectively together to uphold education quality, and ultimately protect public safety. The findings have influenced the GMC's approach to quality assurance which impacts on all medical students and doctors in training.