UK general practice has been described as being in crisis. A shortage and exodus of GPs is an urgent and challenging problem, attracting significant media attention, widespread public debate, and policy action. Our review aims to examine which aspects of the healthcare system affect GP workforce sustainability, how, why, and for whom. A realist review is an interpretive, theory-driven approach to evidence synthesis, that brings together data from quantitative, qualitative, mixed methods research, and the wider grey literature (e.g., policy documentation). Using this data allows us to examine a diverse range of evidence with a clear focus on understanding factors which support (or hinder) GP workforce sustainability, how these are shaped by contexts, and the mechanisms that underpin them. We identify important individual and system-level contexts that may be amenable to change. We present our emerging findings in the form of a programme theory which explores human connection with patients, colleagues, and across organisations, gaining intellectual enrichment and learning systems comprising socially-situated knowledge. Relational continuity is key across these, supporting GP workforce sustainability. Challenges include standardisation, alienation and professional loneliness, inflexible organisation, and restrictive technologies. Our research generates new knowledge about the interdependencies between contexts, mechanisms, and outcomes. The findings can inform strategies and interventions intended to support, facilitate, and assist the GP workforce in delivering equitable and effective patient care. We identify critical gaps in knowledge and prioritise the expectations for scope and nature of future GP work and retention strategies.