Supervision matters: it serves educational, supportive and management functions. Despite a plethora of evidence on the effectiveness of supervision, scant evidence for the impact of supervision training exists. While three previous literature reviews have begun to examine the effectiveness of supervision training, they fail to explore the extent to which supervision training works, for whom, and why. We adopted a realist approach to answer the question: to what extent do supervision training interventions work (or not), for whom and in what circumstances, and why? We conducted a team-based realist synthesis of the supervision training literature focusing on Pawson's five stages: (1) clarifying the scope; (2) determining the search strategy; (3) study selection; (4) data extraction; and (5) data synthesis. We extracted contexts (C), mechanisms (M) and outcomes (O) and CMO configurations from 29 outputs including short (n = 19) and extended-duration (n = 10) supervision training interventions. Irrespective of duration, interventions including mixed pedagogies involving active and/or experiential learning, social learning and protected time served as mechanisms triggering multiple positive supervisor outcomes. Short-duration interventions also led to positive outcomes through mechanisms such as supervisor characteristics, whereas facilitator characteristics was a key mechanism triggering positive and negative outcomes for extended-duration interventions. Disciplinary and organisational contexts were not especially influential. While our realist synthesis builds on previous non-realist literature reviews, our findings extend previous work considerably. Our realist synthesis presents a broader array of outcomes and mechanisms than have been previously identified, and provides novel insights into the causal pathways in which short and extended-duration supervision training interventions produce their effects. Future realist evaluation should explore further any differences between short and extended-duration interventions. Educators are encouraged to prioritize mixed pedagogies, social learning and protected time to maximize the positive supervisor outcomes from training.