Student attrition in postsecondary education is a significant public policy problem. Nations invest substantial resources in college systems, and when students leave, this investment is lost. To understand the factors that influence student attrition in US and Russian public universities, we use the perspective of academic momentum, defined empirically as measures representing student enrollment and study progress. Using a discrete-time event history analysis of samples of eight US and two Russian universities, we provide support for the central claims of the academic momentum theory that undergraduate students who progress through college more rapidly have a lower likelihood of attrition. However, a more detailed analysis reveals variability in the relationship between several academic momentum measures and student attrition, depending on a university's selectivity and the student's chosen academic field and gender.