Abstract : The study presented in this thesis aims to develop knowledge about the process of building a profession. It is based on the case of a hybrid and singular function: the "professeur documentaliste"149. This work constitutes a rupture with the recent scientific publications which are mostly falling within the field of information and communication sciences. Rooted in educational sciences, it takes contributions from several disciplines to understand the interactions between the social representations of different professionals and the work said "actual". It adopts a longitudinal procedure to study the activity of the "professeur documentaliste" with an ergonomic approach. It observes that this atypical teacher is developing identity strategies, influenced sometimes by identification logics, sometimes by differentiation logics towards the disciplinary model. It also notes that, by practising within a teaching system rich in prescriptions, relations and technologies, this teacher regularly has to confront these prescriptions with the complexity of the local situation in order to determine the way to reach the set objectives. Thus, it is through the development of a redefinition of the prescribed task and his/her effective activity that the teacher contributes to the progressive development of an original profession of which he/she builds the features. Through an interactive process between individual and collective conducts, the "professeur documentaliste" participates in structuring his/her professional development and in defining resources which could support the professionalization of the teaching professions in general.This is why this study concludes with a new hypothesis: the one of the revealing characteristic of mutations in teaching professions highlighted by the tribulations of this poorly known and poorly regognised profession.