Online college courses are a rapidly growing feature of higher education. One out of three students now takes at least one course online during their college career, and that share has increased threefold over the past decade (Allen and Seaman 2013). The promise of cost savings, partly through economies of scale, fuels ongoing investments in online education by both public and private institutions (Deming et al. 2015). Nonselective and for-profit institutions, in particular, have aggressively used online courses.In this paper we estimate the effects of taking a college course online, instead of in a traditional in-person classroom setting, on student achievement and progress in college. We examine both mean effects and how online courses change the distribution of student outcomes. While online course-taking is both prevalent and growing, there remains relatively little evidence about how taking a course online, instead