In this book, the author presents several stories that illustrate the forces that shape physician decision-making and may lead to diagnostic mistakes. Borrowing from the field of cognitive psychology, a number of errors stemming from clinicians’ use of heuristics, or “rule of thumb” shortcuts, are highlighted. This book introduced these concepts on a popular level to many clinicians and the public. The book also discusses the role patients can play to minimize these mistakes. A prior AHRQ WebM&M perspective discussed diagnostic errors and provided advice for reducing cognitive slips.