Dual enrollment is becoming increasingly popular as a means to improve student success; however, there is little evidence of its impact on student outcomes. This study, based on analysis of student data from two high school cohorts, examines the causal effects of dual enrollment course taking on marginal students. The author finds that simply taking a dual enrollment course has no impact on these students’ rates of high school graduation, college enrollment, or college degree attainment. However, for marginal students who took a dual enrollment course in college algebra, the impact revealed large and significant effects on college enrollment and graduation rates.