This paper reports on a randomized experiment to study the impact of an alternative teacher preparation program, Teach for America (TFA), on student achievement and other outcomes. We found that TFA teachers had a positive impact on math achievement and no impact on reading achievement. The size of the impact on math scores was about 15 percent of a standard deviation, equivalent to about one month of instruction. The general conclusions did not differ substantially for subgroups of teachers, including novice teachers, or for subgroups of students. We found no impacts on other student outcomes such as attendance, promotion, or disciplinary incidents, but TFA teachers were more likely to report problems with student behavior than were their peers. The findings contradict claims that such programs allowing teachers to bypass the traditional route to the classroom harm students.