This paper develops a conceptual basis for a model on the teaching of socio-scientific controversial issues for secondary or high school students. I argue that the teaching of controversial issues needs a stronger theoretical base. Drawing on a liberal democratic conception of possible sources of conflict, three strands are developed that provide a framework for teachers when teaching socio-scientific issues: these are categories of reasonable disagreement, the communicative virtues, and modes of thought. Examples are given to illustrate how the framework can be used by teachers in which the features of controversy are made explicit to students. (Contains 2 tables and 3 notes.)