This chapter attempts to give the reader an understanding of the underlying theory that enables the design of lenses, with a practical introduction to how the optical industry operates. It provides design techniques for analyzing and designing prism systems. The ultimate cost, shape, and performance of an instrument have as much to do with the design of its optics as with the concept underlying the technique. It is therefore important to understand not just the purpose of the various optical components but also the limitations of a simplistic design approach.