Teachers are important, and policies mandating high-stakes evaluations of teachers are at the forefront of popular school reforms. Today's dominant approach labels teachers as effective or ineffective based in large part on a statistical analysis of students' test-score performance. Teachers judged effective are rewarded, and those found ineffective are sanctioned. While such "summative" evaluations can be useful, lawmakers should be wary of approaches based in large part on test scores: the error in the measurements is large--which results in many teachers being incorrectly labeled as effective or ineffective; relevant test scores are not available for the students taught by most teachers, given that only certain grade levels and subject areas are tested; and the incentives created by high-stakes use of test scores drive undesirable teaching practices such as curriculum narrowing and teaching to the test. This paper presents key research points and advice for policymakers. (Contains 23 notes and references.)