In these uncertain times, politicians and society have been voicing high expectations from teacher education asking for evidence of added value and impact on the student. In this study a longitudinal mixed-methods approach was used to determine the impact of in-service Master of Education programmes on teachers (N = 1,917) and their work environment in the Netherlands. The results of online surveys were explained by means of realist evaluation using focus-group and in-depth interviews. The study shows that Master of Education programmes had an impact on professional and pedagogical skills of teachers, and in some cases also on their work environment. A two-tier mechanism is proposed: the Master's programmes provide teachers with more in-depth knowledge about teaching and learning and a more critical stance through inquiry and research. Consequently, some teachers contribute to a culture of inquiry in their schools, provided that the schools facilitate them in new roles.