In this chapter, some general aspects of designing smaller mirrors are discussed. Common applications are listed to illustrate the use of small mirrors. Next, the design of mirrors is considered from the geometric, image orientation, and system function viewpoints. Methods for determining the required apertures of flat reflecting surfaces for diverging, converging, and collimated beams of given sizes are summarized. The formation of ghost images by second-surface mirrors and a method for estimating their intensities relative to those of the main images are explained. The influences of selected types of reflecting coatings on main image and ghost image intensities are summarized.