Despite rich evidence on the benefit of summer enrollment at the K-12 level, the college completion literature has so far focused on college readiness, remediation, and financial aid, and has largely overlooked the potential benefits of taking summer courses among college students. Academic momentum theory suggests that summer enrollment may increase credit accumulation and retention and thus increase the rate of college completion. Using proximity to the closest four-year college as an instrumental variable (IV), I analyze public higher education data from an anonymous state to examine how enrolling in summer credits can impact college outcomes and the mechanisms by which it may do so. I find that summer enrollees in the sample had higher bachelor's degree completion rates than summer non-enrollees. Summer enrollees returned to college at a higher rate and completed more credits in the following fall without compromising their grade point averages. Students with lower first-term grade point averages benefitted more from summer enrollment. When summer enrollees reached the labor market, they had higher employment rates six years after initial enrollment. Conditional on employment, earnings were equivalent among summer enrollees and non-enrollees. These findings indicate that summer enrollment benefits students through retention, which leads to higher rates of completion and employment. They suggest that colleges may want to seek opportunities for increasing summer enrollment, and they have implications for the current method of Pell Grant allocation, which privileges the fall and spring terms over the summer term.