Clinical reasoning lies at the heart of formulating diagnoses and selecting treatments. The results of these medical decisions determine a substantial portion of the dollars spent on health care. Considering the fundamental importance of clinical reasoning, the topic has received surprisingly little systematic study. Even with the widespread interest in medical error and patient safety in recent years, diagnostic errors and other errors in clinical reasoning have received little attention. This classic collection of empiric studies on clinical reasoning in action thus remains highly relevant more than 25 years after its original publication. One finding of particular relevance for those interested in patient safety and quality improvement is that competence may be problem specific; thus, there is no generic approach to clinical problem solving that, when followed, ensures excellent, or even competent, performance in a variety of domains within a field. The authors also provide an excellent overview of theoretic models relevant to the study of clinical reasoning.