The purpose of this book is to explore the origins, pathophysiology, and diagnostic accuracy of many of the physical signs currently used in adult patients. We have a wonderfully rich tradition of physical diagnosis, and my hope is that this book will help to square this tradition, now almost two centuries old, with the realities of modern diagnosis, which often rely more on technologic tests, such as clinical imaging and laboratory testing. The tension between physical diagnosis and technologic tests has never been greater. Having taught physical diagnosis for 20 years, I frequently observe medical students purchasing textbooks of physical diagnosis during their preclinical years, to study and master traditional physical signs, but then neglecting or even discarding this knowledge during their clinical years, after observing that modern diagnosis often takes place at a distance from the bedside. One can hardly fault a student who, caring for a patient with pneumonia, does not talk seriously about crackles and diminished breath sounds when all of his teachers are focused on the subtleties of the patient’s chest radiograph. Disregard for physical diagnosis also pervades our residency programs, most of which have formal x-ray rounds, pathology rounds, microbiology rounds, and clinical conferences addressing the nuances of laboratory tests. Very few have formal physical diagnosis rounds.