Quartz crystal oscillators were the first frequency and time standards that took advantage of the development of electronic technology. They rely upon the discovery of piezoelectricity by P. Curie in 1880 (Nobel prize 1903) and upon the invention of the first electronic amplifier (the triode) by Lee de Forest in 1907. Such oscillators began to be used in communication systems and in frequency and time metrology laboratories between 1920 and 1930. They were largely used during World war II and, since that time, a large amount of effort has been devoted to improving the resonator design, to optimising the behaviour of the associated electronic circuit and to understanding the physical origin of the short-term frequency instabilities, the sensitivity to external factors and the ageing processes [Besson, 1977; Filler and Vig, 1993; Gerber and Ballato, 1985; Vig, 1991; Walls et al., 1992; Walls and Ggagnepain, 1992]. In the field of interest of this Handbook, high quality quartz crystal oscillators are used as secondary frequency and time standards. Furthermore, they are present in most atomic frequency standards, where their frequency is controlled by the atomic resonance and where they are the source of the output signals.