The use of epitaxial structures for the generation, detection, and modulation of light has had as its strongest driving force the development of fiber optic communications systems. Partly for that reason, the semiconductor materials systems that can be most readily optimized for wavelengths (800-1600nm) transmittable through state-of-the-art optical fibers, have been the most well developed. It also appears that the communications community has been most fortunate in that the semiconductor systems required, although far more difficult to deal with than Si, are relatively benign compared to those that operate at significantly shorter or longer wavelengths. Two developments that are now more than 15 years old initiated the extensive studies of compound semiconductor epitaxial structures that are now so important. These were the introduction of the heterostructure concept in the study of injection lasers and light emitting diodes (1), and the realization that lattice matched isoelectronic epitaxial layers, consisting in part of solid solutions between the binary compounds, would permit such structures to be grown.