The term “professionalization” can both describe the process aiming, on the one hand, at identifying, building, transmitting and developing an expertise characteristic of the profession concerned and, on the other hand, at enabling the construction of a professional identity for the people involved in that process. Our issue questions the conceptions of skilled teaching, explicitly but also implicitly expressed in reports, for the purpose of describing the practice of trainee teachers when observed, analyzing their work and giving them advice. These reports, written up by trainers during the full pedagogic responsibility experience of trainee teachers, are training tools and assessment tools. But they are also the expression of the three categories of leaders’ personal conceptions about primary teaching practice and the professional knowledge at stake. The study presented here was led at the IUFM (Institut universitaire de formation des maîtres) of Rouen in France. It enables to better understand the obstacles in the way of professionalization.