This chapter presents the relationships necessary to determine reflectivities, polarization effects, and so on, of mirrors. It provides the focusing properties of some standard and eminently useful geometries. The chapter discusses the manufacturing errors and limits and their consequences. In recent years a large amount of work has been done on the development of multilayer reflectors for use in the short wavelength region of the spectrum. By measurements of the reflectivity and phase shift of s and p waves it is possible to determine the optical constants. The focusing properties of the Namioka system are similar to those of a plane ellipse and are, as in the case of the plane ellipse, relatively independent of the acceptance of the system, in sharp contrast to a single sphere where coma and spherical aberrations dominate. In addition, an aperture can be located at the intermediate focus in order to mask out unwanted light.