Publisher Summary The chapter discusses imaging properties, and aberrations of spherical optics and nonspherical optics. For spectroscopy in any spectral region, optical components to disperse the radiation into a spectrum are required. Mirrors must be made from (or coated with) a reflective material to achieve a reflectivity high enough to construct a spectroscopic system of reasonable efficiency. Although multilayer coatings can be used for some specific applications, metallic coatings giving broadband reflectivity are most commonly used. The normal incidence reflectivity of metals decreases considerably in the vacuum ultraviolet (VUV) and operation of mirrors at short wavelengths requires small glancing angles. The best reflectors are the heavy metals such as gold and platinum. The chapter discusses the design of extreme ultraviolet (EUV) spectroscopy systems, paraxial optics, geometrical optics with finite apertures and objects, nonaxisymmetrical systems, and so on. Most mirrors for EUV/x-ray spectroscopy, in particular spheres and planes, are made by conventional optical techniques of grinding, polishing, and interferometric testing to the required figure. For applications requiring minimal scattering, a low value of rms surface roughness must be achieved.