This volume contains the lectures and seminars presented at the NATO Advanced Study Institute on "Coherence in Spectroscopy and Modern Physics," the seventh course of the International School of Quantum Electronics, affiliated with the "Ettore Majorana" Centre for Scientific Culture, Erice, Sicily. The Institute was held at Villa LePianore (Lucca), Versilia, Italy, July 17-30, 1977. The International School of Quantum Electronics was started in 1970 with the aim of providing instruction for young researchers and advanced students already engaged in the area of quantum electronics or wishing to switch to this area from a different background. From the outset the School has been under the direction of Prof. F. T. Arecchi, then at the University of Pavia, now at the University of Florence, and Dr. D. Roess of Siemens, Munich. Each year the Directors choose a subject of particular interest, alternating fundamental topics with technological ones, and ask colleagues specifically competent in a given area to take the scientific responsibility for that course.