The general objective of this thesis is to analyze the process by which guidance and career counselors (CO) have experienced developing throughout their working lives, from a professionalization perspective (Uwamariya and Mukamurera, 2005). In recent years, the practice of guidance has evolved a lot in parallel with economic, social and technological transformations which in turn have given rise to new forms of organizations and work-related methods (Cuerrier and Locas, 2004; Mercure, 2018; Thwaites, 2004; Zawieja and Guarnieri, 2014). At the same time, these changes have given rise to a diversification of life courses and professional trajectories (Fournier, Poirel and Lachance, 2016). To adjust to this trend and offer services adapted to the needs of the individuals they support, the CO who participated in this research, as is the case for their colleagues, were invited to constantly renew their practice. in increasingly diverse practice contexts (Ordre des conseillers et conseillères d’orientation du Québec [OCCOQ], 2017). However, in the light of current scientific documentation (Bransford, Brown and Cocking, 2000; Carré and Charbonnier, 2003; Eraut, 2007; Jorro, 2014; Lefeuvre, Garcia and Namolovan, 2009; Marsick, 2006; Rønnestad, Orlinsky, Schröder, Skovholt and Willutzki, 2019), we find that it is still difficult to fully understand the way in which practitioners, and in particular CO, more concretely carry out these professionalization processes during their life course at work. Careful analysis of these writings, however, has made it possible to propose an original conceptualization of these types of process, which can be characterized by dynamic and interactive play between the following three fundamental components: the "actor" (or a practitioner), "the situations that 'it meets in particular environments' and its "activity" (Bourdoncle, 2000; Lefeuvre et al., 2009; Wittorski, 2009). Furthermore, there is little empirical research to better understand the situations that play a significant role in the professional development of practitioners. We also know very little about the learning that can result from these situations, as well as about the possible repercussions of this learning for activity and identity building throughout their professional life. These findings apply particularly to CO, if we consider the little documentation on the subject in the field of counseling and guidance (Cournoyer and Turcotte, 2016; Fournier et al., 2016; Holowiak-Urquhart and Taylor, 2005; Mellouki and Beauchemin, 1994a, 1994b; Osborn and Baggerly, 2004). In addition to the production of knowledge allowing a better understanding of this experience of professionalization, a more in-depth study of these situations and learning and of the role they play in their professional development could also serve as benchmarks for guidance and career counselors and for the training of the future practitioners. In order to examine the process of professionalization of experienced CO, this qualitative research borrows from a narrative / biographical approach (Creswell, 2013; Tochon and Miron, 2004). The sample consists of 30 experienced CO from the practice areas where they are usually found (OCCOQ, 2017). Firstly, the results obtained make it possible to draw up a detailed typology of the situations which played a significant role in the process of professionalization of the participants. The results also made it possible to identify the natures and functions of the learning resulting from the experience of these situations and which influenced their activity and the construction of their professional identity. Secondly, the data reveal that the interaction between these situations and these significant learning offers a relevant framework for understanding the professionalization process among experienced CO. Three typical professionalization profiles have been identified. The discussion then helps to clearly understand the contribution of this thesis for the advancement of knowledge and leads, in conclusion, to prospects for training and research.