This report describes the results of a study of teachers’ thinking about data. It looks closely at the teacher preparation and support necessary to implement data-driven instruction. Understanding the nature of teachers’ proficiencies and difficulties in data use is important for providing appropriate training and support to teachers. Through the use of case scenarios, the report provides a detailed description of teacher thinking about data use that can help inform those responsible for professional development and induction. It showcases the difficulties and misconceptions inherent in early data use practices, and in this way is an invaluable tool for those developing strategies for large-scale support of teachers’ use of educational data.