Written by a distinguished physicist and leading researcher, this volume describes the theory and selected applications of one of the most important mathematical tools used in the theoreticial investigation of collective excitations in statistical physics. The text offers an introduction to functional integral techniques in equilibrium statistical physics, and discusses the expression of partition functions and Green functions in terms of functional integrals. Sections of the book deal with the application of functional integrals in superfluid Bose systems, systems with Coulomb interaction, and superfluid Fermi systems. The book also considers the application of the concept of Bose-condensation of auxiliary fields to the theory of crystals, heavy atoms and also to the theory of model Hamiltonians.