What data should teachers use, and how should they use these data to improve their teaching? In this review, a diverse set of literature that addresses this question is categorized according to three distinct approaches: the diagnostic approach, the methods approach, and the teacher approach. Within the diagnostic approach, teachers collect data to determine the students who need to learn specific content. Within the methods approach teachers collect data to determine the aspects of teaching that contribute to student learning. Within the teacher approach, teachers collect data to help the teacher develop beliefs and knowledge that support effective teaching. Benefits and drawbacks of these approaches are identified, and some guidelines and cautions concerning the use of data to improve instruction are discussed.