This paper explores some widespread assumptions about knowledge, and particularly the disciplinary or subject knowledge that is construed as an essential property of effective teachers. Drawing on close observation of pre-service teachers' work in secondary school classrooms, it argues that knowledge is born from social engagement and has its life in social activity; thus to conceptualise knowledge as the possession of the teacher, to be passed on to learners, is misleading and unhelpful. What is required is a much more complex, nuanced understanding of the knowledge work that is accomplished in and through the interactions of the classroom.