Book excerpt: This work reviews the principles underlying quantum-based atomic clocks, with introductory chapters placing them in context with the development of mechanical clocks and electronic quartz-controlled clocks. The book details design principles of the rubidium, cesium, hydrogen maser, and mercury ion standards; changes enabled by the advent of the laser; and the time-based global navigation systems, Loran-C and the Global Positioning System. The new edition includes such recent developments as clocks based on quantum resonance at optical frequency in individual ions confined in miniature electromagnetic traps. The Quantum Beat explores the subject with a minimum of mathematical formalism.