This chapter addresses theory and practice in a developmental research project in which mathematics teachers and didacticians worked together to develop mathematics teaching. The mathematics teachers were from eight schools in Norway, ranging from lower primary to upper secondary. Didacticians were academics in mathematics education in a university. Both were practitioners in their own areas of practice, and in the project both were researchers. The project sought to know more about how mathematics teaching can develop to enhance learning experiences for students in mathematics classrooms. It was called Learning Communities in Mathematics (LCM)1 and involved teachers and didacticians in inquiry communities exploring together and evaluating possibilities for classrooms and students (Jaworski et al., 2007). Over a period of 4 years, including three phases of school-related activity each of one school year, 14 didacticians (including fi ve doctoral students) and 35 teachers were involved in the project.