Teacher education in Germany is a complex system comprising various phases, training institutions, and stakeholders. Throughout their training, prospective teachers form social relationships with a wide range of people. With some, they have official training relationships (e.g., instructors, practicum supervisors). Others are not official trainers of the student teachers, yet they are still part of their daily training routine (e.g., fellow students or student teachers). These official and unofficial training stakeholders form a social network of relationships that significantly influence the professionalization of prospective teachers. These training actors, the social relationships with them, and their influence on professionalization have already been identified to some extent as subjects of research. However, there has been a lack of a network-analytical perspective that examines 1) the formation and change of networks and 2) the interaction among actors with regard to professionalization. This dissertation study focuses on the second phase of teacher education and aims to examine the social networks of student teachers during their practical training. The goal is to analyze how these social networks form and evolve over the course of the training, as well as the professionalization effects resulting from relationships and interactions with training stakeholders. To this end, as part of a qualitative longitudinal study, 15 student teachers were accompanied during their practical training, and network interviews were conducted at three different time points. These were evaluated using a combination of content-structuring qualitative content analysis and qualitative structural analysis. The results show that the formation and transformation processes of social networks among student teachers can be explained by five factors: perceived freedom of action, perceived professional support, perceived emotional and social support, perceived network burdens, and perceived function/role of training stakeholders. Furthermore, the professionalization effects could be explained using the two mechanisms of social learning and social pressure. For both mechanisms, different types of reactions and coping strategies were also identified, which can lead to effects that either promote or inhibit professionalization.