Discussion is widely held to be the pedagogical approach most appropriate to the exploration of controversial issues in the classroom, but surprisingly little attention has been given to the questions of why it is the preferred approach and how best to facilitate it. Here we address ourselves to both questions. We begin by clarifying the concept of discussion and justifying it as an approach to the teaching of controversial issues. We then report on a recent empirical study of the Perspectives on Science AS-level course, focusing on what it revealed about aids and impediments to discussion of controversial ethical issues.