In recent years, the sociology of education has seen a renewed interest in realist accounts of knowledge and its place in education. Inspired by "social realist" thinking, a body of work has emerged that criticises the dominance of generic and process-based thinking about (especially) professional education and advocates instead a revaluation of discipline-based and theoretical knowledge. In this paper, I discuss the role of the concept of expertise in professional education. Following Winch, I situate the dominant theories of expertise in the field today as "fluency" accounts of expertise -- such theories focus more on the fluency or automaticity with which the expert acts than on the content of what the expert can do. As an alternative, I investigate Collins and Evans's recent work on expertise in the sociology of scientific knowledge. Similar to what Collins and Evans suggests for science studies, I hold that education would benefit from consideration of the developing "third wave" of thinking about the nature of expertise and I sketch the main features of a social realist view of the nature of expertise for professional education.