The specification of various categories of knowledge that teachers should possess has been a historically consistent feature of moves to professionalize school teaching and to argue for individual teachers' professional autonomy. In this article, I suggest that the ways in which subject knowledge has been treated in research-based recipes for teachers' professional knowledge are often characterized to various degrees by three epistemological problems: the problem of dualism; the problem of objectivism; and the problem of individualism. In place of dualistic, individualistic and objectivist typologies, the article proposes a realistic alternative: a situated view of subject knowledge as emergent within complex and dynamic social systems. A model of this developmental process is offered that represents the development of subject knowledge in practice, that is, teaching a subject in schools. The article concludes with a brief discussion of the practical implications of this view of subject knowledge and teacher development for teacher education programmes. (Contains 2 figures and 6 notes.)