In this theoretical essay, we identify the specificities of the second career teachers’ entry process into schools by proposing a reflection and a conceptualization that define it as a process of learning through and at work, or more specifically as a work socialization process. In this sense, this paper contributes to the reflections on the links between work, training, and professionalization by proposing a case study in which training and professionalization processes are fundamentally situated in the work activity itself. Learning, training, and professionalization, therefore, occur “on the job”, in the same temporality, which in this case is the first steps of second career teachers in their new work environment.