NANDA has traditionally complemented its work on the development and clinical testing of nursing diagnoses with concurrent interest in nurses' diagnostic reasoning processes. This literature review describes studies in diagnostic reasoning, including research based on the information processing model, research examining the role of intuition, and research using grounded theory to study the diagnosis of specific patient conditions. The review presents a synthesis of the research to date, and common themes in these diverse studies are identified. This review emphasizes research in nursing, although pertinent findings from cognitive psychology and medicine are included. Suggestions for future work in diagnostic reasoning are offered.