When choosing optimum materials for system components, the optical designer must consider many material characteristics. Optical parameters as well as structural and environmental requirements must be specified and toleranced. System performance and operating and storage requirements must be generated first to ensure that the optics contribute their required functional share. For example, whether or not focus of an objective or beam divergence of an afocal can be passively maintained by athermalization or by servo-driven feedback can largely depend on the selection of optical materials and their mechanical mountings. Laser damage thresholds may require inclusion-free materials and low scattering surfaces. These sur-faces can be another driver of substrate selection. Continued efforts in uniformly and more completely specifying optical materials are required to achieve a more predictable level of excellence in performance of EO systems. This paper provides both an overview of optical materials and application-oriented considerations for their selection.