Institutional diversity, or the existence of many different kinds of colleges and universities within a specific higher education system, has long been recognized as a positive and unique attribute of the U.S. higher education system. This paper describes a study that examines the change in institutional diversity across all 50 states between two snapshot years, 1972 and 2002. The study uses institutional theory to conceptualize the identified changes. IPEDS (Integrated Postsecondary Educational Datasets) data on institutional characteristics and enrollment datasets from the two study years are used to identify growth or decline in specific college and university types. Institutional types are categorized according to factors identified as important for researchers and consumers of higher education (e.g., size, degree level, control, cost) in an attempt to highlight what institutional types have become more or less populated during the study period. The paper also includes a brief discussion of academic drift, which serves to highlight the importance of determining the causes of changes in the institutional diversity of higher education systems. The goal of this study is to assess whether institutional theory and its propositions provide a serviceable framework for understanding changes in the institutional diversity of U.S. higher education. (Contains 9 tables and 10 notes.)