In this article, John Papay argues that teacher evaluation tools should be assessed not only on their ability to measure teacher performance accurately, but also on how well they inform and support ongoing teacher development. He looks at two major approaches to teacher evaluation reform: value-added measures and standards-based evaluations. Papay analyzes these two approaches both as measurement tools and as professional development tools, illuminating the advantages, drawbacks, and untapped potential of each. In the process, attention is refocused towards a broader conception of the purpose of teacher evaluation. (Contains 7 notes.)