In this chapter, we first address the geometric relationships that govern the function of prisms and then consider specific designs for prisms that serve useful purposes in optical instruments, but do not contribute optical power and, hence, cannot form images by themselves. Small motions or misalignments of some such components relative to other system components may not affect overall system performance. Examples are a Porro prism that tilts about an axis perpendicular to the plane of reflection and a cube-corner prism that tilts about any of three orthogonal axes. In both cases, for small tilts of the optic, the reflected beams are not disturbed. Prisms with this characteristic are called constant-deviation prisms with respect to those particular axes.