In this research we explore the how the distribution of teacher qualifications and student achievement in New York City have changed from 2000 through 2005 using data on teachers and students. We find: the gap between the qualifications of New York City teachers in high-poverty schools and low-poverty schools has narrowed substantially over this period, the gap-narrowing associated with new hires has been driven almost entirely by the substitution of teachers entering through alternative certification routes, for uncertified teachers in high-poverty schools, these changes resulted from a direct policy intervention eliminating unlicensed teachers, and perhaps most intriguing, much larger gains could result if teachers with strong teacher qualifications could be recruited.