Criteria sets formalize our approach to studying the etiology, course, and management of rheumatic diseases, and provide a conceptual base for measuring future improvements in clinical care. They focus our clinical objectives and improve our clinical research activities. They are dynamic, evolving, and will certainly undergo major changes. Understanding the purposes of specific criteria sets and the differences between different criteria categories is crucial for understanding the rheumatic disease literature and for the design and conduct of clinical and epidemiologic investigations.