This paper discusses the materials issues of acousto-optic (AO) devices. Criteria for the selection of materials for the three basic types of AO devices, deflectors (Bragg cells), modulators, and tunable filters, are presented. Comparison of physical properties of the various AO materials shows that trade-offs are required to suit specific applications. As an example of how to search for new materials, the potential of a chalcopyrite compound (ZnGeP2) for AO device applications is discussed. The paper includes tables that list the properties of selected AO materials.