CONTEXT This paper presents a realist synthesis of the literature that began with the objective of developing a theory of workplace learning specific to postgraduate medical education (PME). As the review progressed, we focused on informal learning between trainee and senior doctor or supervisor, asking what mechanisms occur between trainee and senior doctor that lead to the outcomes of PME, and what contexts shape the operation of these mechanisms and the outcomes they produce? METHODS We followed the procedures outlined in the RAMESES Publication Standards for Realist Synthesis. We searched the English-language literature published between 1995 and 2017 for empirical papers related to informal workplace learning between supervisor and trainee, excluding formal interventions such as workplace-based assessment. We made a pragmatic decision to exclude general practice training to keep the review within manageable limits. RESULTS We reviewed 5197 papers and selected 90. Synthesis revealed three workplace learning processes occurring between supervisors and trainees, each underpinned by a pair of mechanisms: supervised participation in practice (entrustment and support seeking); mutual observation of practice (monitoring and modelling), and dialogue during practice (meaning making and feedback). These mechanisms result in outcomes of PME, including safe participation in practice, learning skills, attitudes and behaviours and professional identity development. Contexts shaping the outcomes of these mechanisms were identified at individual, interpersonal, local and systems levels. CONCLUSIONS Our realist theory of workplace learning between supervisors and trainees is informed by theory and empirical research. It highlights the two-way nature of supervision, the importance of trainees’ agency in their own learning and the deleterious effect of fragmented working patterns on supervisor trainee learning mechanisms. Further empirical research is required to test and refine this theory. In the meantime, it provides a useful framework for the design of supportive learning environments and for the preparation of supervisors and trainees for their roles in workplace learning.