There is considerable debate about the best way to measure and improve teacher quality. One method is to rate teachers based on their students’ test score gains, known as the “value-added” approach. School districts have begun to use these measures to make personnel decisions about teachers. For example, District of Columbia Public Schools lay teachers off or offer them bonuses using a teacher-performance assessment system that puts 50 percent weight on value-added scores. This article describes an extensive study designed to estimate the effects of teachers on student outcomes through early adulthood.