IntroductionThe real‐world mechanisms underlying prospective entrustment decision making (PEDM) by entrustment or clinical competency committees (E/CCCs) are poorly understood. To advance understanding in this area, the authors conducted a realist synthesis of the published literature to address the following research question: In E/CCC efforts to make defensible prospective entrustment decisions (PEDs), what works, for whom, under what circumstances and why?MethodsRealist work seeks to understand the contexts (C), mechanisms (M) and outcomes (O) that explain how and why things work (or do not). In the authors' study, contexts included individual E/CCC members, E/CCC structures and processes, and training programmes. The outcome (i.e. desired outcome) was a PED. Mechanisms were a substantial focus of the analysis and informed the core findings. To define a final corpus of 52 included papers, the authors searched four databases, screened all results from those searches and performed a full‐text review of a subset of screened papers. Data extraction focused on developing context–mechanism–outcome configurations from the papers, which were used to create a theory for how PEDM leads to PEDs.ResultsPEDM is often driven by default (non‐deliberate) decision making rather than a deliberate process of deciding whether a trainee should be entrusted or not. When defaulting, some E/CCCs find red flags that sometimes lead to being more deliberate with decision making. E/CCCs that seek to be deliberate describe PEDM that can be effortful (when data are insufficient or incongruent) or effortless (when data are robust and tell a congruent story about a trainee). Both information about trainee trustworthiness and the sufficiency of data about trainee performance influence PEDM. Several moderators influence what is considered to be sufficient data, how trustworthiness data are viewed and how PEDM is carried out. These include perceived consequences and associated risks, E/CCC member trust propensity, E/CCC member personal knowledge of and experience with trainees and E/CCC structures and processes.DiscussionPEDM is rarely deliberate but should be. Data about trainee trustworthiness are foundational to making PEDs. Bias, equity and fairness are nearly absent from the papers in this synthesis, and future efforts must seek to advance understanding and practice regarding the roles of bias, equity and fairness in PEDM.