The aim of this article is to extend the scope of the models of teacher change to an interactionist view which co-ordinates sociocultural and constructivist perspectives. Accordingly our focus will be on the cultural and situational factors and processes of social interaction, as well as on the development on an individual level. First we study how roles, statuses, expectations as well as social and socio-mathematical norms were formed in the group of four student teachers, when they were co-planning, teaching and assessing mathematics lessons during their teaching practice. Finally, we attempt to construct a collaborative, interactionist model of teacher change integrating sociocultural and constructivist perspectives.