This article reviews the experimental and theoretical studies of the properties of hydrogen included in metals. The problem is discussed of diffusion of hydrogen atoms in a metallic matrix, and the theory of coherent and incoherent diffusion. Phase transitions caused by elastic interactions are considered in a system of hydrogen atoms included in the interstices of a metallic matrix. The electronic properties of compounds of hydrogen with different metals, the experimental studies of the electronic band structure and of phase transitions in the electronic subsystem (metal-dielectric in cerium hydride, magnetic transitions) are reviewed. Special attention is paid to superconductivity in hydrogen-metal systems. Detailed results are presented of experimental and theoretical studies of the superconducting properties of compounds of palladium with hydrogen and deuterium, and also compounds of palladium with the noble metals and hydrogen. The effect of inclusion of hydrogen on the superconductivity of other transition metals is discussed.