As the financial challenges facing the U.S higher education industry mount, colleges and universities seek new activities that can improve their financial situation. Online education programs are often viewed as a promising option due to growing student interest and the substantial net revenue generated by early entrants that leveraged economies of scale and scope. The number of schools that can experience similar outcomes will depend upon whether the online market is primarily concentrated, with a small number of higher education institutions enrolling most students, rather than fragmented, with large numbers of institutions enrolling meaningful amounts of online students. We examine this topic by providing the first detailed study of market concentration in online education. We first introduce a theory from economics, Sutton's model of endogenous fixed costs, to the higher education literature to highlight key dynamics that shape concentration levels. Using institution-level enrollment data for the population of Title IV postsecondary institutions, we then empirically examine concentration levels in online education. We find that relative to in-person education, national online enrollment patterns are heavily concentrated. The online market has a distinctive structure, consisting of a small number of large national providers and a large number of small local providers. Online enrollment patterns became less concentrated between Fall 2012 and Fall 2018 due to major enrollment declines of large for-profit providers. Concentration levels, however, plateaued by the end of the period due to rapid enrollment growth at large national nonprofit providers.