The aim of this book is to provide a wealth of information on the influence of quantum-tunneling systems on the low-temperature properties of solids. The nineteen contributors of this book review thermodynamic, acoustic, dielectric and optical properties of amorphous and crystalline solids in eight chapters. The newest experiments on long-time spectral diffusion and heat release, electric field and strain effects, as well as nonequilibrium phenomena are thoroughly discussed. Several theoretical contributions provide an overview of the most original ideas and theoretical methods used nowadays to understand the nature and dynamics oftunneling systems in solids and their interactions. The book provides full description on the experimental and theoretical details of the relevant experiments and theories. Within the broad spectrum of properties that are related to the contribu­ tion of tunneling systems, I have selected special topics which will be of value for experienced as well as graduate students working on low-temperature re­ search. Most of the chapters serve as useful introductory surveys, since they are not too specialized. I wish to express my thanks to the following colleagues for their impor­ tant comments on several chapters of this book: Ansel C. Anderson, Guy Bellessa, Dimitri Parshin, Roman Personov, Robert Silbey, James Skinner, Ulrich Weiss, and Helmut Wipf. I would like to thank Mrs. Annette Setzer for her help with the reference list, as well as to the co-workers at the De­ partment of Superconductivity and Magnetism, Universitiit Leipzig, for their support.