Although well researched in educational studies, inquiry-based learning, a student-centred way of teaching, is far away from being implemented in day-to-day science and mathematics teaching on a large scale. It is a challenge for teachers to adopt this new way of teaching in an often not supportive school context. Therefore it is important to provide high-quality professional development (PD) at a large scale. However, there is little empirical evidence about the effects of scaled-up professional development initiatives. Therefore, this paper presents an international research study for which long-term PD courses have been designed. These courses have been implemented across Europe in twelve different countries at scale by using the so-called "Cascade Model." Here, course leaders are educated, who in turn educate other teachers. The research study aimed at evaluating the overall impact the scaled-up PD had on teachers and at also identifying variables influencing this impact.