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. It also recognizes, however, that the value-added framework does not address all potential impediments to consistently estimating the quality of instruction. Specific methods mitigate some problems and not others; none may resolve all potential problems. Acquiring a clearer understanding of these issues can improve the methods used to estimate added teacher value and how these estimates are used. The first section describes student, teacher, and principal choices that are primary determinants in student-teacher matching. The following section outlines a cumulative model of learning and considers the advantages of various estimation methods given the student-teacher matching process and test measurement error and structure. The final section focuses on implications for policy. It begins by describing the advantages and disadvantages of specific methods before turning to a more general discussion of the use of value-added estimates in the teacher evaluation and compensation. (Contains 3 notes.)