This volume contains ten lectures presented in the series ULB Lectures in Nonlinear Optics at the Universite Libre de Bruxelles during the period October 28 to November 4, 1991. A large part of the first six lectures is taken from material prepared for a book of somewhat larger scope which will be published, by Springer under the title Quantum Statistical Methods in Quantum Optics. The principal reason for the early publication of the present volume...