This important research explores the effects of district policy interventions on the distribution of teacher qualifications and student achievement. Authors use a 5-year span of individual teacher- and student-level longitudinal data from New York City (NYC) from 2000 through 2005 to estimate the differences in the effectiveness of teachers entering NYC schools through different pathways to teaching. The study finds that the gap between the qualifications of NYC teachers in high-poverty and low-poverty NYC schools has narrowed substantially since 2000, mostly ensuing from the city's concentrated effort to match exceptionally capable teachers with very needy students and the virtual substitution of newly hired uncertified teachers in high-poverty schools with new hires from alternative certification routes: NYC Teaching Fellows and Teach for America.