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...