This article assesses the status of diachronic research on the United States Congress. A literature review reveals a lack of truly diachronic studies, but a wealth of insightful and useful historical and contemporary studies. Drawing on organization theory, a research approach designed to facilitate and improve diachronic analysis is put forward. A major focus of the approach is on the ways in which environmental factors—both fixed and variable—shape the operations and performance of Congress. In addition, problems and strategies in analyzing the impacts of Congress on the wider political system are examined. The concluding section presents a research agenda for political scientists interested in applying diachronic analysis to the study of Congress.