X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wuerzburg University in Germany. Working with a cathode-ray tube in his laboratory, Roentgen observed a fluorescent glow of crystals on a table near his tube. Roentgen called this new radiation X-radiation. The "X" in x-ray stands for unknown.
The first human x-ray Roentgen took was of his wife's hand just one week after discovering x-rays! In 1913 he invented the Coolidge tube, a X-ray tube with an improved cathode for use in X-ray machines that allowed for more intense visualization of deep-seated anatomy and tumors.