This article is originated from a research program that focuses on the professional identity formation of preservice teachers. We will show how this process takes place in a teacher training curriculum. The theoretical and methodological framework is based on the clinical analysis of activity theory (Clot, 1999). Results show the importance, for the preservice teachers, to renormalize the prescriptions and to develop their professional identity. They highlight the difficulties that preservice teachers have to take into account prescriptions, in their activity, and to become teachers.