This research investigates the relative effectiveness (in terms of student tested achievement) of Teach for America (TFA) teachers, and examines the validity of the criticisms of TFA. Specifically, the authors look at TFA teachers in secondary schools, and especially in math and science, where considerable program growth is planned over the next few years. Using individual level student data linked to teacher data in North Carolina, the authors estimate the effects of having a TFA teacher compared to a traditional teacher on student performance. The North Carolina data they employ are uniquely suited for this type of analysis because it includes end of course (EOC) testing for students across multiple subjects. This allows them to employ statistical methods that attempt to account for the nonrandom nature of student assignments to classes/teachers, which have been shown to lead to biased estimates of the impact of teacher credentials (Clotfelter, Ladd, and Vigdor, 2007a; Goldhaber, 2007). The findings show that TFA teachers are more effective, as measured by student exam performance, than traditional teachers. Moreover, the authors suggest that the TFA effect, at least in the grades and subjects investigated, exceeds the impact of additional years of experience, implying that TFA teachers are more effective than experienced secondary school teachers. The positive TFA results are robust across subject areas, but are particularly strong for math and science classes. An appendix is included. (Contains 1 figure, 12 tables, 1 exhibit and 14 footnotes.) [This report was supported by the Steven L. Merrill Family Foundation.]