The aim of this study was to find out how science teachers who have participated in a one-year school-based collaborative teacher professional development programme, perceive the programme's impact on their professional development. Constant comparative analysis was used on data from three schools to generate the findings in this study. The results indicate that participating teachers perceived a clear effect on their teaching practice and ways of thinking about teaching science. In addition, the findings identify a positive effect on the teachers' attitudes towards collaboration. The article further discusses how reflection seems to contribute to the teachers' professional development.