This paper presents an international research study of long-term professional development courses on modelling. It addresses the question of scaling-up professional development. So far, there has been much research on small-scale professional development courses, but we know very little about what it means to scale up such a course and to reach out to large numbers of teachers. Therefore, our study researches the impact of a scaled up professional development course on teachers and their teaching, as perceived by the teachers themselves and their students. The course was designed on an international level for use in 12 countries. The results show that such a course can indeed lead to desired outcomes concerning the teachers and their teaching, and the research therefore adds to our understanding of scaling-up.