The concept is simple and attractive: Evaluate teachers on the basis of how much academic growth their students experience over the course of the school year. Often referred to as "value-added," this concept and the statistical methods for implementing it have been a topic of debate in state legislatures and at state and national education conferences over the past decade. Recently, the concept and the practice have also been catching on in schools, districts and states across the country. Results from value-added models are already playing an increasing role in the process of identifying teachers in need of targeted professional development. But, as is often the case, the issues involved in implementing this seemingly straightforward idea are complex and pose both statistical and practical challenges. In this Policy Information Perspective, the author examines value-added models and concludes with advice for policymakers who are seeking to understand both the potential and the limitations inherent in using such models to evaluate teachers. While welcoming the possibility of introducing a quantitative component into the teacher evaluation process, the author counsels policymakers to move forward with caution, especially if high stakes are attached to the results. (Contains 48 footnotes.)