Optical frequency measurement is an extremely challenging field of experimental physics which is presently undergoing a renaissance driven by the needs of modern high density optical communication systems as well as by requirements of high-resolution laser spectroscopy. This text is the first to discuss the development of traditional and second generation frequency chains together with their enabling technology. Reviews written by some of the most experienced researchers in their respective fields address the technology of frequency metrology such as: low noise microwave oscillators and microwave frequency standards, low noise and high stability optical frequency sources, optical frequency standards, traditional and second-generation optical frequency measurement and synthesis techniques as well as optical frequency comb generators. It should prove useful to researchers just entering the field of frequency metrology or equally well to the experienced practitioner.