We are currently witnessing an increase of international interest in mathematics education, fueled partly by the growing concerns of students’ declining results, but also by changed perceptions of what mathematics students should master. In response, many initiatives have appeared in order to move away from traditional to more inquiry based approaches to teaching. Though several small-scale studies have contributed much to our understanding on how to support teachers in this work, there is still a lack of research conducted on a larger scale. Therefore, the aim of this thesis is to add to our knowledge of how to support instructional improvement at scale. This is done by focusing on two common approaches to support mathematics teachers’ development of reform based practices: teacher professional development [PD] programs and curriculum materials. The thesis builds on four papers which are all connected to a project aiming at improving the mathematics instruction in a large Swedish municipality. The project includes a PD-program for almost 400 elementary teachers and the mathematics curriculum materials that teachers are using play a central role in the program. The first two papers focus on curriculum materials either by using surveys to compare teachers’ views of the support offered in the materials and their reported mathematics instruction, or by conducting textbook analyses to characterize how some commonly used materials communicate about, for example, goals of lessons. The results demonstrate that teachers using different materials experience different levels of support from them and also show variations in their reported instruction. These differences are further reflected in the textbook analyses which show that the materials offer teachers various support, for example regarding how they communicate about goals. The last two papers focus on teacher PD-programs either by comparing the effects of two programs on student achievement, or by using surveys to examine teachers’ views of one of the programs and its impact on their reported instruction. The results indicate that the two PD-programs have affected students’ achievement in different ways, demonstrating both decline and improvement. Even within the programs differences are revealed between students at the primary and secondary levels. These variations are further present in the teacher surveys, where the results show differences between teachers from different grade-levels. By drawing on the literature review and the results of the papers, the thesis ends with a discussion of possible elaborations of a widely used core conceptual framework for studying teacher PD.