Value-added modeling offers the possibility of estimating the effects of teachers and schools on student performance, a potentially important contribution in the current environment of concern for accountability in education. These techniques, however, are susceptible to a number of sources of bias, depending on decisions about how the modeling is executed and on the quality of the data on which models are based. If teachers are to be held accountable for the performance of their students, strategies for measuring the impact of their work must be refined or, at least, the uncertainties of these measurements must be taken into account in assessing the impact of teachers and schools on student performance. [This research brief describes work done for RAND Education documented in "Evaluating Value-Added Models for Teacher Accountability" by Daniel F. McCaffrey, Daniel M. Koretz, J. R. Lockwood, and Laura S. Hamilton.]