In 1922 Edward Thorndike wrote, "The task of education is to make changes in human beings. For mastery in this task, one needs definite and exact knowledge of what changes are made and what ought to be made". Seventy-nine years after he wrote these words, the 2001 No Child Left Behind (NCLB) legislation stated, "Schools must have clear, measurable goals focused on basic skills and essential knowledge. Annual testing in every grade gives teachers, parents and policy makers the information they need to ensure that every child will reach academic success". While the language of the two may be different, the message is the same: schools need accurate and actionable information about what students know and can do so that they can plan effectively for student learning. Despite the contemporary prominence of achievement data as vehicles to school improvement and research evidence showing that data use is a characteristic of effective schools and districts, there remain substantial challenges to the meaningful integration of data use into the everyday practices of schools. This article examines characteristics of effective data use, outlines challenges in the implementation of school-based data analysis, and offers perspectives as to how best practice in data use can become a widespread reality in schools.