We use six years of panel data on students and teachers to evaluate the effectiveness of recently hired teachers in the New York City public schools. On average, the initial certification status of a teacher has small impacts on student test performance. However, among those with the same experience and certification status, there are large and persistent differences in teacher effectiveness. Such evidence suggests that classroom performance during the first two years is a more reliable indicator of a teacher's future effectiveness. We also evaluate turnover among teachers by initial certification status, and the implied impact on student achievement of hiring teachers with predictably high turnover. Given modest estimates of the payoff to experience, even high turnover groups (such as Teach for America participants) would have to be only slightly more effective in each year to offset the negative effects of their high exit rates (I2, J24). (Contains 3 figures and 7 tables.)