Until recently, states focused on ensuring the presence of a "highly qualified teacher" in every classroom. Under the 2001 reauthorization of the Elementary and Secondary Education Act (ESEA), known as No Child Left Behind (NCLB), this title described a teacher holding at least a bachelor's degree and the appropriate state license and demonstrating subject matter competency. Research has shown that these "input" measures do little to explain differences in student performance. Thus, in recent years, the conversation has shifted to a focus on "outputs"--how effective a teacher is at improving student achievement. Knowing how well, or how poorly, educators are performing is critical to drive improvement strategies for individual teachers, schools, districts, and states and to inform accountability systems. High-quality teacher evaluation data can also be used to inform policies across the education system, including measuring the effectiveness of teacher preparation programs, informing performance-based compensation, ensuring students have equal access to highly effective teachers, and identifying professional development needs. The need for quality data on teacher performance is clear, and the reform of teacher evaluation systems must be considered in the context of other activities designed to improve educator effectiveness. This issue of re:VISION, part of a special series on teacher effectiveness, examines the evolution of teacher evaluation systems and the most commonly used evaluation measures, and offers considerations for policymakers who are examining teacher evaluation in their states. A glossary of common student achievement measures is provided. [For Part 1 of this series, see ED559385; for Part 3, see ED559381; for Part 4, see ED559388; and for Part 5, see ED559391.]