Arguably the most important educational resource is teachers. Teachers and teaching quality are a central feature of the No Child Left Behind Act of 2001 (NCLB) which requires a "highly qualified teacher" in every core academic classroom. Many states and large districts also have policies in place to attract qualified teachers to difficult-to-staff schools. NCLB, state assessment-based accountability policies and new routes into teaching have all had profound effects on the labor market for teachers. In this research, the authors explore the effect of these changes on the distribution of teacher qualifications and student achievement. This analysis employs individual student and teacher-level data for grades 3 through 8 for each year from 2000 through 2005. This paper suggests that selection of teachers with stronger qualifications has made an important difference in New York City public schools and that recruitment and retention of teachers with stronger measurable characteristics can lead to improved student learning. (Contains 3 figures.) [This research is part of the activities of the National Center for the Analysis of Longitudinal Data in Education Research (CALDER).]