Recent advances in the quantum theory of macroscopic systems have brightened up the field and brought it into the focus of a general community in natural sciences. The fundamental concepts, methods and applications including the most recent developments, previously covered for the most part only in the original literature, are presented here in a comprehensive treatment to an audience who is reasonably familiar with quantum-statistical mechanics and has had rudimentary contacts with the path integral formulation. This book deals with the phenomena and theory of decoherence and dissipation in quantum mechanics that arise from the interaction with the environment. A general path integral description of equilibrium thermodynamics and non-equilibrium dynamics is developed. The approach can deal with weak and strong dissipation, and with all kinds of memory effects. Applications to numerous phenomenological and microscopic systems are presented, where emphasis is put on condensed matter and chemical physics. The basic principles and methods of preparation functions, propagating functions, and time correlation functions are described. Special attention is focused on quantum tunneling and quantum coherence phenomena of macroscopic variables. Many illustrative realistic examples are discussed in some detail. The book attempts to provide a broad perspective and to open up this rapidly developing field to interested researchers normally working in different fields. In this enlarged second edition, the nineteen chapters of the first edition have been expanded by about one-third to better meet both the requests of newcomers to the field and of advanced readers, and seven new chapters have beenadded that review the most recent important developments.