A century ago, the living body, like most of the material world, was opaque. Then Wilhelm Roentgen captured and X-ray image of his wife's finger—her wedding ring “floating” around a white bone—and our range of vision changed forever. By the 1920s, X-ray technology was common-place: all army recruits had lined up for chest pictures during WWI, and children were examining the bones of their feet in shoe store fluoroscopes, spectacularly unaware of the radiation they were absorbing. Through lucid prose, vivid anecdotes, and over seventy striking illustrations, science writer Bettyann Holtzman Kevles shows how X-rays and the subsequent daughter technologies—CT, MRI, PET, ultrasound—transformed the practice of medicine (from pediatrics to neurosurgery), the rules of evidence in courts, and the vision of artists.