Policymakers and school administrators have embraced value-added models of teacher effectiveness as tools for educational improvement. Teacher value-added estimates may be viewed as complicated scores of a certain kind. This suggests using a test validation model to examine their reliability and validity. Validation begins with an interpretive argument for inferences or actions based on value-added scores. That argument addresses (a) the meaning of the scores themselves--whether they measure the intended construct; (b) their generalizability--whether the results are stable from year to year or using different student tests, for example; and (c) the relation of value-added scores to broader notions of teacher effectiveness--whether teachers' effectiveness in raising test scores can serve as a proxy for other aspects of teaching quality. Next, the interpretive argument directs attention to rationales for the expected benefits of particular value-added score uses or interpretations, as well as plausible unintended consequences. This kind of systematic analysis raises serious questions about some popular policy prescriptions based on teacher value-added scores.