The utility of value-added estimates of teachers' effects on student test scores depends on whether they can distinguish between high- and low-productivity teachers and predict future teacher performance. This article studies the year-to-year variability in value-added measures for elementary and middle school mathematics teachers from five large Florida school districts. We find year-to-year correlations in value-added measures in the range of 0.2-0.5 for elementary school and 0.3-0.7 for middle school teachers. Much of the variation in measured teacher performance (roughly 30-60 percent) is due to sampling error from "noise" in student test scores. Persistent teacher effects account for about 50 percent of the variation not due to noise for elementary teachers and about 70 percent for middle school teachers. The remaining variance is due to teacher-level time-varying factors, but little of it is explained by observed teacher characteristics. Averaging estimates from two years greatly improves their ability to predict future performance.