Riehle (optics, Physikalisch-Technische Bundesanstalt, Germany) describes the central methods and techniques relevant to frequency standards developed in physics, electronics, quantum electronics, and statistics. He first reviews basic concepts of frequency standards from the microwave to the optical level, then discusses the realization of commonly used components, offering practical examples. He describes the most important frequency standards, such as the rubidium clock, and presents various applications of frequency standards in high-technology areas.