In education, 'constructivism' constitutes the 'grand unified theory' of the moment. In this article, I maintain that constructivism as a theory of knowledge and constructivism as pedagogy are distinct and that the question of what constructivism about knowledge implies for teaching is under-theorised. Seven classroom scenarios are sketched that illustrate the problems that a constructivist view of knowledge can create in the classroom. It is concluded that constructivist epistemology undermines effective teaching; as such, realistic teaching practice cannot proceed from constructivist assumptions regarding the nature of knowledge. The conclusion, however, is neutral regarding teaching practice: constructivist epistemology is neither sufficient nor necessary for what is called 'constructivist' teaching practice.