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Improvements in astronomical time measurement techniques and in the definition of time have kept pace with developments in frequency standards. Quartz crystal frequency standards are described, including Essen rings, bars, GT-cut plates, and contoured AT-cut plates. Stable oscillator circuits for quartz-crystal frequency standards are described, including the Meacham bridge-stabilized circuit, the Gouriet-Clapp circuit, and the Lea quartz-resonator-servo circuit. A discussion of the present status of atomic and molecular frequency standards includes the ammonia absorption cell, ammonia oscillator (MASER), and cesium atomic-beam apparatus. Instrumentation for precision frequency measurement is outlined, and a current listing of standard-frequency broadcast stations is included.