This chapter concerns the importance of in-service training programmes for the professional development of science teachers. A description will be given of the general structure of in-service training activities in Turkey and the results will be presented of an in-service training project, which was conducted as part of a three-year longitudinal study. Within the scope of the project, an in-service training programme for science teachers was conducted based on the argument-based inquiry approach and the theoretical premises upon which it is built. The project aimed to direct science teachers towards student-centred teaching. The training activities focused on the scientific thinking underpinning the teachers' professional knowledge and practices, their perceptions on learning, and their pedagogical practices and epistemological beliefs. The extent to which these activities affected classroom applications and learning processes was investigated and the findings suggest that they had a statistically significant impact on the teachers' pedagogical development and on the students' academic performance and thinking skills.