General education requirements comprise, on average, approximately 30% of the undergraduate curriculum and therefore represent an important feature of the student academic experience in American colleges and universities. Previous studies have not fully examined the origins of the most important models of general education, the distribution of these models among higher education institutions, or the causes for change in general education requirements over time. In this study, the authors describe and analyze the organization of general education requirements in U.S. four-year colleges and universities over a 25-year period, 1975-2000. They show that four models of general education persisted throughout the period. They will label these the "core distribution areas," "traditional liberal arts," "cultures and ethics," and "civic/utilitarian" models. They show that two of these models arose near the beginning date of the study. The rise of these new models is one important change in general education. Another is the addition of new subject requirements, particularly in areas related to basic academic skills, gender and racial-ethnic diversity, and non-Western cultures. These course-level changes, the authors will argue, reflect responses to the deeper-lying forces of expansion and diversification of higher education, as interpreted by influential actors in the system. The authors conclude by discussing the implications of their study for the further development of institutional theory, and particularly for the analysis of institutional change. (Contains 3 tables, 2 figures and 15 notes.)