Optical fiber transmission lines now appear to be competitive with copper wires andcoaxial cables in certain applications. In addition, aplications not otherwise feasiblemay employ optical fibers. Considerable research and development effort has been de-voted to producing silica fibers with extremely low loss (<1 dB/km)as well as to devisingcomponets and techniques required to operate a fiber system. Established fiber-opticaltechnology will be employed in a communication system by the Bell System in 1980.