This third edition, motivated by the numerous and significant developments in the laser field since the publication of the second edition in 1982, is a substantially revised version of the previous edition. The basic philosophy has, however, remained the same, namely, to provide a broad and unified descrip­ tion of laser behavior at the simplest level that is compatible with a correct physical understanding. The basic organization of the book has also remained the same. The book is therefore aimed at both classroom teaching and self-study by students in electrical engineering, physics, and chemistry who have an interest in understanding the principles of laser operation. The major additions to this edition are the following: 1. New sections dealing with laser types, in particular x-ray lasers and new solid-state lasers, including alexandrite devices, and a greatly extended description of semiconductor lasers. 2. A more extended treatment of laser mode-locking, including new sections on cavity dumping and pulse compression. 3. A more extended and greatly simplified description of the coherence and statistical properties of laser light as opposed to those of conven­ tional light. 4. A greatly extended discussion of the physics of gas discharges. Other important additions include a discussion of some topics from conven­ tional optics (e.g., ray matrix methods, Fabry-Perot interferometers, and multilayer dielectric mirrors), Gaussian beam propagation (e.g., the ABeD law), and the theory of relaxation oscillations and active mode-locking.