As outlined in the general introduction given in Chap. 1 of Vol. I, the theoretical description of light was revolutionized in the 1960s through the development of quantum optics, pioneered by R. J. Glauber [1–3], and statistical optics largely developed by E. Wolf and L. Mandel [4, 5]. Mandel also made many pioneering experimental contributions to quantum optics. Both treatments abandon the deterministic nature of stable classical waves for the concept of probability amplitudes, following the earlier lead by Max Born. The theoretical development occurred in the wake of the development of the maser and laser in the late 1950s and the Hanbury Brown–Twiss experiment in 1956 [9–11].