The development of optical thin films for waveguide applications will enable a variety of new and highly useful optoelectronic functions. Many difficulties must be overcome, however, before optical films can be used in photonics applications. Properties such as polycrystallinity, surface topography, and refractive-index homogeneity can affect the performance of optical thin film waveguides to a much greater extent than in other thin film applications because of the long optical interaction lengths. Even a small degree of polycrystallinity in anisotropic media, for example, can be a significant source of optical scattering. We review recent results of epitaxial lithium tantalate thin films grown as waveguide structures on sapphire and gallium arsenide substrates. Highly oriented crystalline structures with useful optical properties have been demonstrated. Although many challenges still remain before optical thin films are used widely as waveguides, modem thin film technology is beginning to provide the tools and capabilities to make these concepts a reality.