"Based on a series of lectures given by the author in 1969 at the Brandeis University Summer Institute in Theoretical Physics. The material was published initially as an article entitled 'Some aspects of the physics of gas lasers', in Atomic physics and astrophysics, vol. 2, pp. 1-201, edited by M. Chretien and E. Lipworth )Gordon and Breach, New York, 1973(."