Metal hydrides are of inestimable importance for the future of hydrogen energy. This unique monograph presents a clear and comprehensive description of the bulk properties of the metal-hydrogen system. The statistical thermodynamics is treated over a very wide range of pressure, temperature and composition. Another prominent feature of the book is its elucidation of the quantum mechanical behaviour of interstitial hydrogen atoms, including their states of motion. The important topic of hydrogen interaction with lattice defects, and its materials-science implications, is also discussed thoroughly. This second edition has been substantially revised and updated.