Inquiry-based learning (IBL) is a more student-centered approach to mathematics teaching that is recommended by many policy and curriculum documents across Europe. However, it is not easy for teachers to change from a more teacher-centered way of teaching to inquiry-based teaching as this involves a change of their role in class. Professional development courses are one way to help teachers with this endeavor. Within the discussion of effective professional development, beliefs are often named as an important influencing factor. In this respect, much research has been carried out on how beliefs on mathematics teaching impact the outcomes of the course. However, there has been much less research on what beliefs mathematics teachers develop on inquiry-based learning and how this might impact their (perceived) classroom teaching. Therefore, this paper presents an international research study carried out within the European Project Primas, in which professional development courses on inquiry-based learning were conducted in 12 countries. Using the case-study approach, this paper aims at answering the following questions: 1. What kind of beliefs about IBL do mathematics teachers across Europe develop? 2. How do these beliefs relate to teachers' perceived enactments of IBL?