One hundred years ago this month Wilhelm Ro¨ntgen, a professor of physics at the Julius Maximilian University of Wu¨rzburg, discovered x rays while experimenting with cathode rays in a Crookes tube. Word of the discovery spread quickly, and by early 1896 the properties of x rays were under investigation in numerous physics laboratories in Europe and North America. By the turn of the century, physicians and physicists were exploiting the penetrating character of x rays to look inside the human body without cutting it open. They were also beginning to explore the therapeutic properties of x radiation.