[en] The roles of crystal structure, hydrogen site occupancy, phase transition, and isotope substitution on the hydrogen diffusion behavior will be considered. After brief discussions of general diffusion concepts and the experimental methods that have been most often used to determine the hydrogen mobility in the high concentration metal hydride phases, the nuclear magnetic resonance techniques that have been extensively applied in hydride diffusion studies will be reviewed in some detail. The emphasis will be on relating measured nuclear relaxation times with diffusion parameters as well as NMR techniques to directly measure the hydrogen diffusion constants. The diversity of diffusion behavior will be illustrated with several specific metal hydride systems including PdH/sub x/, γ-TiH/sub x/ and structurally related TiCuH, and VH/sub x/. Plausible models of the microscopic diffusion mechanisms will also be presented and compared with the observed diffusion behavior in these systems