The spectra of stars, even for early-type (hot) stars, fall off steeply in the ultraviolet. The falloff is roughly according to the Wien radiation law that prevails on the short-wavelength side of the black-body radiation peak. Nevertheless, there has been considerable interest in the ultraviolet because many spectral features pertaining to ionized atoms lie in that region, and so help in studying the stars. Because our atmosphere becomes quite opaque to hinder the study of ultraviolet and X-rays, they held a high priority for incipient space astronomy. Then, during a rather modest rocket mission to look for X-rays from solar wind or cosmic bombardment of the moon, the discovery of a powerful point-like souce in the constellation Scorpius came much to the surprise of the astronomical community. That discovery heralded much more to come, all of which begs for X-ray telescopes. The problem is that materials do not refract or reflect X-rays, so that we cannot just extend optical designs to the X-ray region. The lack of reflection is a slight overstatement since grazing incidence reflection can be had, and often is used for soft X-rays.