The evaluation of teachers based on the contribution they make to the learning of their students, value-added, is an increasingly popular but controversial education reform policy. We highlight and try to clarify four areas of confusion about value-added. The first is between value-added information and the uses to which it can be put. One can, for example, be in favor of an evaluation system that includes value-added information without endorsing the release to the public of value-added data on individual teachers. The second is between the consequences for teachers vs. those for students of classifying and misclassifying teachers as effective or ineffective — the interests of students are not always perfectly congruent with those of teachers. The third is between the reliability of value-added measures of teacher performance and the standards for evaluations in other fields — value-added scores for individual teachers turn out to be about as reliable as performance assessments used elsewhere for high stakes decisions. The fourth is between the reliability of teacher evaluation systems that include value-added vs. those that do not — ignoring value-added typically lowers the reliability of personnel decisions about teachers. We conclude that value-added data has an important role to play in teacher evaluation systems, but that there is much to be learned about how best to use value-added information in human resource decisions.