This brief describes estimation and measurement issues relevant to estimating the quality of instruction in the context of a cumulative model of learning. It also discusses implications for the use of value-added estimates in personnel and compensation matters. The discussion highlights the importance of accounting for student differences and the advantages of focusing on student achievement gains as opposed to differences in test scores. Despite potential shortcomings, value-added analysis can provide valuable information for use in evaluating and compensating teachers. The key is not to be cavalier about the information contained in value-added estimates but to understand the pieces that go into producing estimates of teacher quality.