This book provides a wealth of information on the influence of quantum-tunneling systems on the low-temperature properties of solids. It reviews the thermodynamic, acoustic, dielectric and optical properties of amorphous and crystalline solids. The latest experiments on long-time spectral diffusion and heat release, electric field and strain effects, quantum tunneling on light atoms like H and D, as well as non-equilibrium phenomena are thoroughly discussed in terms of the interaction with and between tunneling systems. Several outstanding theoretical contributions provide an overview of the most original ideas and theoretical methods used to understand the nature and dynamics of tunneling systems in solids and their interactions. The book introduces the non-expert reader to the phenomenological standard tunneling model and its further development, the soft potential model. The chapters of the book give full descriptions of the experimental and theoretical methods of the relevant experiments and models in the fascinating field of the physics of glasses and disordered solids.