In this introductory article, after some initial discussion of an appropriate approach to mathexnatics as a curriculura subject, we sketch a theory for designing teaching, based on mathematical activity, situations, tasks, and interventions, exposing and resolving cognitive conflicts, changes of structure and context, feedback, reflection and review. We next review the main psychological principles underlying this theory, then consider some examples of teaching designs in the light of the theory. Thus we open the discussion of the theme of this issue, which continues with the fuller discussion of other examples in the remaining articles.Other authors have offered more controversial descriptions, which emphasize the logical aspects. One may recall Russell's "the subject in which we never know what we are talking about, nor whether what we say is true". For the purposes of guiding curriculum construction, we find the positive descriptions more helpful.Education is normally seen as a forward-looking, purposeful activity, the aim of which is to develop pupils' capacities and knowledge so as to equip them