The major classes of raw materials from which optical instruments are created include glasses, plastics, crystals, semiconductors, ceramics, metals, thin films of all the aforementioned materials, composites, adhesives, sealing compounds, and special finishes. In this chapter, we consider for each of these classes the general characteristics of importance in opto-mechanical systems design and tabulate typical values for selected parameters and materials. Sources in the literature for more detailed information regarding these and other materials of interest are cited. Just as Wolfe (1990) reported for refractive properties of infrared (IR) materials, the mechanical properties of most types of materials used in optical instruments can vary slightly from lot to lot and from one source to another. 97Numerical values cited here are approximate and may not be adequate for final design purposes. Materials manufacturers should be contacted for specific values pertaining to their products, and if adequate data are not available, measurements should be made on representative samples.