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The availability of coherent sources producing usable amounts of power in the optical frequency range has stimulated considerable research in optical communications. Devices such as oscillators, modulators, detectors, and ancillary apparatus having desirable characteristics exist and are being used to design and build prototype terminals. Two possible media are being studied and means are being sought to improve their performance. They are 1) through-the-atmosphere propagation and 2) enclosed media with appropriate focusing and directing elements. Experimental optical transmission systems can readily be assembled with information capacities in a single RF channel comparable to those of microwave radio or millimeter waveguide. Such optical systems are not yet competitive for high reliability common carrier service because 1) long-distance transmission techniques of adequate reliability have not yet been advanced, and 2) optical repeater components are not yet competitive with their lower frequency counterparts. Some features characteristic of optical transmission systems are reviewed in this paper, along with a brief indication of the state-of-the-art for major components.