Mounting pressure in the policy arena to improve teacher productivity either by improving signals that predict teacher performance or through creating incentive contracts based on performance--has spurred two related questions: Are there important determinants of teacher productivity that are not captured by teacher credentials but that can be measured by subjective assessments? And would evaluating teachers based on a combination of subjective assessments and student outcomes more accurately gauge teacher performance than student test scores alone? Using data from a midsize Florida school district, this paper explores both questions by calculating teachers' "value added" and comparing those outcomes with subjective ratings of teachers by school principals. Teacher value-added and principals' subjective ratings are positively correlated and principals' evaluations are better predictors of a teacher's value added than traditional approaches to teacher compensation focused on experience and formal education. In settings where schools are judged on student test scores, teachers' ability to raise those scores is important to principals, as reflected in their subjective teacher ratings. Also, teachers' subject knowledge, teaching skill, and intelligence are most closely associated with both the overall subjective teacher ratings and the teacher value added. Finally, while past teacher value added predicts future teacher value added the principals' subjective ratings can provide additional information and substantially increase predictive power. (Contains 9 tables and 15 footnotes.)