A growing body of literature suggests that when schools become professional communities there are expected benefits in terms of teacher learning, school improvement and student achievement. In this article the concept of professional communities is examined for certain subject departments in Dutch secondary schools. The authors report on research into the extent to which mathematics departments operate as professional communities. At the same time, it was investigated whether the level at which departments operate as professional communities is related to student achievement. The results indicate that departments are cohesive bodies regulating teacher behaviour in several respects. However, as professional communities they do not focus on improving the quality of their teachers and instruction. Some characteristics of professional communities prove to be beneficial for student achievement, while others are not. The authors offer recommendations on how departments can develop into more professionally organised communities.