We review the key elements from 1895 to the Second World War, that characterize the fantastic history of "radiology". This term encompasses initially both imaging and treatment, as very rapidly the therapeutic potential of radiation has been exploited. Although the medical usefulness of the rays has quickly been suspected, it took quite a while to put in place the first rules in radioprotection, with the pioneers paying the high price of these repetitive exposures to ionizing irradiation. Radioactivity has even been popularized and marketed on a large scale, because it was attributed a multitude of imaginary virtues, until the real dangers were finally understood. The years around the Second World War accelerated research and technological development in the context of a frantic race by the belligerents to develop the atomic bomb first. It's the American project called «Manhattan» which won. However, all this research during the war will ultimately lead to substantial progress in the medical field.