This book is motivated by the very favorable reception given to the previous editions as well as by the considerable range of new developments in the laser eld since the publication of the third edition in 1989. These new developmentsinclude, among others, Quantum-Well and Multiple-Quantum Well lasers, diode-pumpedsolid-state lasers, new concepts for both stable and unstable resonators, femtosecond lasers, ultra-high-brightness lasers etc. The basic aim of the book has remained the same, namely to providea broad and unied description of laser behavior at the simplest level which is compatible with a correct physical understanding. The book is thereforeintendedas a text-bookfor a senior-levelor rst-year graduatecourse andor as a reference book. This edition corrects several errors introduced in the previous edition. The most relevant additions or changes to since the third edition can be summarized as follows 1. A much-more detailed description of Amplied Spontaneous Emission has been g