Data-based decision making can help teachers improve their instruction. Research shows that instruction has a strong impact on students' learning outcomes. This study investigates whether Dutch primary school teachers use data to improve their instruction. Four aspects of instruction were distinguished: purposeful teaching, adaptive instruction, feedback and learning time. Data were collected by means of a survey (n=318) and through interviews with teachers (n=18). The results show that although almost all teachers use data with the intention of improving their instruction, they skip important steps in the data use process. They do not make optimal use of all available data and fail to carry out all the relevant analyses. Teachers mainly use data when their own students have disappointing learning outcomes. They are, however, less interested in using data related to the functioning of the school as a whole.