The purpose of this paper is to present a model of how teachers might perform in classrooms and to discuss certain consequences that it has for the training and work of the teacher. We argue that individual differences in teaching competence derive in part from how teachers have adapted their skills, abilities and values to the demands and responsibilities of the teaching settings in which they have worked during their careers. It is proposed that teacher-training programmes could use an understanding of how these changes occur as a source of principles to guide the selection of content for and the organization of their curricula.