Abstract X-ray astronomy—the study of astronomical bodies by the X-rays they emit—started only after the second World War. This is because the radiation is absorbed by the atmosphere, and can be observed only with instruments carried by balloons or rockets. This article describes the present state of the study. An important fraction of sunspot activity has been found to consist of X-rays, and in addition, a number of ‘X-ray stars’ have been discovered. There is also a background of diffuse X-rays which may originate from near the edge of the observable universe.