In recent years, increasing emphasis has come to be placed, both by researchers and by policy-makers, on the adequacy of primary teachers' subject knowledge of science. Within research, this emphasis has been linked to the rise of constructivist ideas about the significance of establishing children's prior conceptions of scientific concepts for effective teaching. In this article, I examine two constructivist approaches to teachers' adequacy of subject knowledge within UK research on primary science education. In each case, I provide a critique of the assumptions they make about the nature of knowledge and how it develops. I do this from a sociocultural perspective, which views knowledge and learning as necessarily situated within communities of practice. My aim is to assess the implications of this perspective for understanding teachers' adequacy of subject knowledge. (Contains 4 notes.)