While doctoral education and postgraduate supervision might be heavily researched in the context of "effective supervision", this is still an acutely under-theorised field. This article responds to the call for more explicit theorising of postgraduate supervision by exploring doctoral education as academic subjectification, and supervision as a process of "category boundary work". It is argued that postgraduate supervision entails a relationship in which the boundaries around what constitutes culturally intelligible academic performativity, "academicity", are negotiated, maintained, challenged and reconstructed. The concept of category boundary work is put to work in a brief analysis of the ways in which it is performed by, respectively, a supervisor and supervisee within the social sciences. The article aims to provide a perspective and an analytical tool which can be applied in context-sensitive ways, across disciplinary, institutional, biographic and national settings, to produce significant insight into how academic cultures and subjectivities are re/produced.