Secondary school teaching is organized in departments and effective departments functioning as collaborative teams have been associated with effective schools. Therefore, this study investigates the relationship of mathematics departments perceived as professional communities and student achievement in Dutch secondary schools. Cluster analysis and hierarchical linear modeling (HLM) were used on a sample of around 3000 students, 130 schools, and 130 teachers that participated in this study (TIMSS-2003 data). The significant effects found (effect size greater than 0.20) show that those departments that focus on reflective dialogue, collaborative activity, shared vision and student achievement are associated with successful schools and higher student achievement.