X-rays are so powerful that they tend to pierce right through typical telescope mirrors if they strike them head-on. Therefore, X-ray telescopes use nested layers of “grazing-incidence mirrors” that reflect X-rays along very shallow angles. The need for grazing incidence optics makes X-ray telescopes very challenging to design. (Compared to optical telescopes, they often look like they are pointed backward!) Furthermore, X-rays do not go through the atmosphere well, so all X-ray telescopes must be space telescopes. The scientific reward, however, is well worth the difficulty of building them. X-ray telescopes afford astronomers an opportunity to study directly some of the most energetic phenomena in the universe, like novae, supernovae, pulsars, and black holes.