The photoelastic constants describe the effect of stress or strain on the refractive indices of materials. These coefficients are important in several applications. They are required for the computation of stress-induced optical distortion in optical systems such as high-power laser systems and they are needed for computing figures of merit for materials to be used in acousto-optic devices. Interferometric and polarimetric techniques are described for measuring piezo-optic coefficients under static load-ing conditions. Acousto-optic and Brillouin scattering techniques are described for measuring elasto-optic constants.