This study assessed the extent to which four principal performance measures based on student test scores—average achievement, school value-added, adjusted average achievement, and adjusted school value-added—accurately reflect principals' contributions to student achievement in future years. Average achievement used information on students' end-of-year achievement without taking into account the students' past achievement; school value-added accounted for students' own past achievement by measuring their growth; and adjusted average achievement and adjusted school value-added credited principals if their schools' average achievement and value-added, respectively, exceeded predictions based on the schools' past performance on those same measures. The study conducted two sets of analyses using Pennsylvania's statewide data on students and principals from 2007/08 to 2013/14. First, using data on 2,424 principals, the study assessed the extent to which ratings from each measure are stable by examining the association between principals' ratings from earlier and later years. Second, using data on 123 principals, the study examined the relationship between the stable part of each principal's rating and his or her contributions to student achievement in future years. Based on results from both analyses, the study simulated each measure's accuracy for predicting principals' contributions to student achievement in the following year. The study found that the two performance measures that did not account for students' past achievement—average achievement and adjusted average achievement—provided no information for predicting principals' contributions to student achievement in the following year. The two performance measures that accounted for students' past achievement—school value-added and adjusted school value-added—provided, at most, a small amount of information for predicting principals' contributions in the following year, with less than one-third of each difference in value-added ratings across principals reflecting differences in their future contributions. These findings suggest that principal evaluation systems should emphasize measures that were found to provide at least some information about principals' future contributions: school value-added or adjusted school value-added. However, study findings also indicate that even the value-added measures will often be inaccurate in identifying principals who will contribute effectively or ineffectively to student achievement in future years. Therefore, states and districts should exercise caution when using these measures to make major decisions about principals and seek to identify nontest measures that can accurately predict principals' future contributions.