In the complex environment of 21st-century public education, a consensus has emerged among policymakers, researchers, and educational practitioners that the single most important factor contributing to student learning is the quality of teaching. Despite this unanimity in principle, the practical matter of guaranteeing high-quality instruction for every student in every classroom poses a serious challenge with no simple solution in sight. Embedded in conversations about teacher preparation, induction, and professional development are dilemmas about how to properly support teachers and develop their capacities and skills while holding them accountable so that the best will stay and the worst are persuaded to leave.Calls for evidence of a more effective teacher workforce from policymakers, federal agencies, state departments of education, and local school districts have resulted in a trend of increasingly stringent accountability measures. Some fairly recent formulations-for example, the "report card" requirements for teacher preparation programs included in the 1998 reauthorization of the Higher Education Act of 1965 and the "highly qualified teacher" provision of the No Child Left Behind Act of 2001-emphasize punitive consequences for weak results. In the case of the Higher Education Act, teacher preparation programs that are judged to be "low performing" lose their accreditation. Similarly, No