Every space-borne optical system has a unique set of requirements and challenges. In the end, all need to achieve a delicate balance of performance, cost, and schedule to be successful. Principal to achieving this goal is the correct selection of the mirror materials from among a variety of common options: glass, aluminum, beryllium, and silicon carbide. Many engineering disciplines -- optical, mechanical, thermal, and materials -- are required to design an optical system. The intent of this paper is to describe the strengths and limitations of each mirror material from the perspective of an optical systems engineer, who must balance all disciplines in order to select the best material combination for a specific application. Several topics are discussed, including: (1) Mechanical and thermal figures of merit; (2) stability and design considerations (K/α, ΔL/L, and cryogenic test data); (3) fabrication issues: schedules, relative costs, achievable figures, and finishes; and (4) stray light rejection and achievable BRDF's. Data is presented from many recent projects in support of these topics.