Both clinicians and patients rely on an accurate diagnostic process to identify the correct illness and craft a treatment plan. Achieving improved diagnostic accuracy also fulfills organizational fiscal, safety, and legal objectives. It is frequently assumed that clinical experience and knowledge are sufficient to improve a clinician's diagnostic ability, but studies from fields where decision making and judgment are optimized suggest that additional effort beyond daily work is required for excellence. This article reviews the cognitive psychology of diagnostic reasoning and proposes steps that clinicians and health care systems can take to improve diagnostic accuracy.