This study uses administrative data linking students and teachers at the classroom level to estimate teacher value-added to student test scores. We find that variation in teacher quality is an important contributor to student achievement--more important than has been implied by previous work. This result is attributable, at least in part, to the lack of a ceiling effect in the testing instrument used to measure teacher quality. We also show that teacher qualifications are almost entirely unable to predict value-added. Motivated by this result, we consider whether it is feasible to incorporate value-added into evaluation or merit pay programs. (Contains 11 tables, 2 figures and 43 footnotes.)