Clinical reasoning allows physicians to move from areas of clinical uncertainty to points where the medical literature offers guidance, and is equally important in deducing whether the results of clinical trials are applicable to an individual patient. However, studies in the field of cognitive psychology indicate that the reasoning skills of clinicians are imperfect. Moreover, clinicians may be aware of their mistakes but often do not understand the cognitive processes underlying their errors. Greater understanding of the reasoning process has the potential to improve patient care but independent study of clinical reasoning can be difficult, as the literature is complex and unfamiliar to most physicians. This article provides an introduction to diagnostic reasoning and highlights some of the cognitive factors that lead to errors in clinical problem solving. Clinical scenarios are used to illustrate key points and place the material in a readily accessible framework.