In this policy brief, the author explores the problems with attainment measures when it comes to evaluating performance at the school level, and explores the best uses of value-added measures. These value-added measures, the author writes, are useful for sorting out-of-school influences from school influences or from teacher performance, giving everyone overall better performance measures. Value-added measures provide summative assessments of teacher performance. They indicate whether teachers are doing well or not, on one important measure of student performance. But value-added is often criticized for not providing information about "how to improve". Value-added measures can be made more "formative" in this sense by providing multiple value-added measures for teachers on the subject or specific domains covered by tests. No single measure can fulfill both the formative and summative functions very well, however. For this reason, any use of value-added, especially for individual teachers, should be coupled with observational information from school principals or peer assessors. Multiple measures provide more information, and more information is generally better. The issue is not whether to use value-added, but how to do so. (Contains 3 figures and 21 endnotes.)