Teach For America (TFA) recruits high-achieving college graduates to teach for 2 years in the nation's low-income schools. This study is the first to examine these teachers' retention nationwide, asking whether, when, and why they voluntarily transfer from their low-income placement schools or leave teaching altogether. Based on a survey of three entire TFA cohorts (n = 2,029), this longitudinal, retrospective study uses discrete-time survival analysis. We found that teachers who have more challenging assignments--split grades, multiple subjects, or out-of-field classes--are at greater risk of leaving their schools or resigning from teaching than those with single-grade, single-subject, or in-field assignments. It is notable that in-field science teachers' risk of resigning was higher than that of their out-of-field counterparts with nonscience degrees. This study informs policymakers and school officials seeking to retain TFA and other promising teachers. (Contains 7 notes and 4 tables.)