Improving teacher preparation is critical to long-term improvement in teacher quality. More than 200,000 new teachers enter classrooms each year. Increasing student enrollment, the retirement of baby boom generation teachers, and high attrition in their first five years (between 40 and 50 percent of new teachers leave the profession) have transformed the teacher workforce. As a result, first-year teachers are now the single largest cohort each year. While these numbers might cause alarm, they also highlight a huge opportunity to improve teacher effectiveness. Because states set requirements for certification and licensure and have the power to approve both traditional and alternative preparation programs, the means to transform teacher preparation is well within reach. Policymakers know that improving teaching in our schools requires a systematic review of the many policies that impact educator effectiveness. For example, to be successful, improvements in teacher preparation must be complemented by reforms in educator evaluation, compensation, and school leadership. This issue of re:VISION, part of a series on teacher effectiveness, examines teacher preparation and offers considerations for policymakers in this important area of reform. [For Part 1 of this series, see ED559385; for Part 2, see ED559387; for Part 3, see ED559381; and for Part 5, see ED559391.]