Schools face a lot of data on the functioning of their school which they can use to make improvements in teaching, learning and the organization. For data use to lead to improvement, it is important to further research the concept data-driven decision making. The results of this explorative study in the Netherlands show that teachers mainly use classroom level data for making instructional decisions at classroom level, and school leaders mainly use school level data for policy development decisions. This article ends with suggestions with regard to enhancing the effectiveness of data-driven decision making, for example by stressing the importance of developing teachers' competence in the use of data. (Contains 3 tables and 1 figure.)