The doctorate is an educative process for students but what is its impact on supervisors' learning about the practice of doctoral supervision? Internationally, there is an increased emphasis on formal training, monitoring and accountability of doctoral supervisors. Yet there is a striking silence about what doctoral supervisors learn through supervising doctoral students, and how the impacts on supervisors might be theorised. The aim of this article is to begin to address this gap in the doctoral education literature, based on a thematic analysis of two complementary interview studies of a cross-disciplinary sample of experienced doctoral supervisors. The analysis illustrates the significant impact of doctoral supervision on the learning and knowledge of doctoral supervisors, particularly in relation to how supervisors engage with/in the social and political context of their university, understand themselves and their students, and how the contemporary context of supervision affects the sort of pedagogical relationships supervisors establish with their doctoral students. Regardless of supervisors' discipline, position in the academic hierarchy or supervisory experience, the analysis indicates that supervisors' learning experiences shape their subjectivities and identities, and that supervision is an ongoing ontological process of ‘becoming a supervisor’. The importance of integrating a theory of ‘becoming a supervisor’ into supervisor professional development is proposed.