Context General practice supervisors are said to serve as the cornerstones of general practice postgraduate education and therefore it is important to clearly define their roles and what makes them effective. The commonly used definition of a supervisor is not primarily based on general practice and does not cover aspects predicted to be important according to work-based learning theory. Methods We searched for papers published between 1991 and 2011 inclusive, categorised them according to whether they provided empirical evidence, descriptions or recommendations, open-coded the empirical evidence, and used the resulting coding scheme as an analytic framework within which to present a narrative summary of findings. Results Recommendations and descriptions far outweighed empirical evidence, which showed how supervisors intertwined clinical and educational activities and formed educational alliances with resident doctors that provided a foundation for learning. Residents needed a balance of challenge, usually provided by patients, and support, provided by supervisors. Supervisors established learning environments, assessed residents' learning needs, facilitated learning, monitored the content and process of learning and the well-being of residents, and summarised learning in ways that turned 'know that' into 'know how'. Conclusions General practice must be expert in ensuring patients are well cared for 'by proxy' and in giving residents just the right amount of support they need to face the challenges posed by those patients. As general practice responds to contemporary clinical demands and rising numbers of undergraduate medical students, it is essential that the ability of general practice supervisors to develop and sustain supportive supervisory relationships with residents is preserved.