This paper is part of the ongoing work of the author and others in developing a social realist theory of knowledge for educational studies. It contrasts Durkheim and Vygotsky's theories and why both are important for educational theory. It begins by emphasizing the similarities between them; that knowledge has to be understood in terms of its historical development in human societies and that the acquisition of knowledge is the primary goal of formal education. In contrasting the ideas of the two writers the paper develops the distinction between "structure" for Durkheim and "activity" for Vygotsky and explores some of the strengths and weaknesses of each approach. It then examines how the ideas of the two writers have been developed by their leading followers in sociology and psychology--giving particular consideration to the work of Basil Bernstein and Yrjo Engestrom. It concludes with some broader issues concerning theories of knowledge in educational studies. (Contains 11 notes.)