Material properties for selection in optomechanics are those that influence the shape and position of the optical surfaces of a system. Optical, mechanical or structural, and thermal properties are of interest. Cost is also important and is usually related to the ease of fabrication. Typically, materials are chosen based on figures of merit (FOMs), which are combinations of properties related to certain kinds of performance. Care is necessary to avoid the “single property fallacy,” in which a material is chosen based on a single performance parameter without consideration of other possibly adverse properties. This chapter examines optical FOMs, structural FOMs, thermal FOMs, the strength of brittle optical materials, optical glasses, optical plastics, infrared materials, mirror substrate materials, and material cost.