The main objectives in building professional learning communities are to improve teachers' professionalism and well-being, and create positive impacts on student learning. It is a question of changing the school culture. The main objective of this quantitative study was to investigate the maturity level of thirteen Finnish schools as professional learning communities from the perspectives of school culture, leadership, teaching, and professional development. The participants' perceptions indicated a culture of collegiality, trust and commitment as common strengths at all schools. The school cultures supported professional collaboration, and the teachers had the knowledge, skills and dispositions to engage in professional collaboration. The challenges were related to structural conditions, especially the lack of collaboration time. Three school profiles were identified in the cluster analysis from the viewpoint of maturity as professional learning communities. Statistically significant differences between the three clusters were found in organizational and operational characteristics.