As part of the Bill & Melinda Gates Foundation's broader efforts to improve educational opportunities for all students, the "Teachers Know Best" research project seeks to encourage innovation in K-12 education by helping product developers and those who procure resources for teachers better understand teachers' views. The original "Teachers Know Best" report, released in 2014, was an attempt to share with educators and product developers detailed information about how K-12 teachers and students view digital instructional tools as a whole. This follow-up study focuses on the potential of a specific subset of digital instructional tools: those that help teachers collect and make use of student data to tailor and improve instruction for individual students. The use of data is a crucial component in personalized learning, which ensures that student learning experiences--what they learn and how, when, and where they learn it--are tailored to their individual needs, skills, and interests and enable them to take ownership of their learning. Personalized learning is critical to meeting all students where they are, so they are neither bored with assignments that are too easy nor overwhelmed by work that is too hard. The intent of "Making Data Work" is to drill down to help educators, school leaders, and product developers better understand the challenges teachers face when working with this critical segment of digital instructional tools. More than 4,600 teachers from a nationally representative sample were surveyed about their use of data to drive instruction and the use of these tools. They were asked to talk about their behaviors and beliefs, to discuss specific occasions in which they use data to guide student learning, to describe and rate current tools, and to talk about the challenges they face, and offer advice on ways to make digital tools more effective for teachers and students. In addition, the study included qualitative research in innovative schools that are at the forefront of personalizing learning for students, as well as in more traditional public schools. The results of the study as well as recommendations for product developers, teachers, district and school leaders, and funders are included in the report.