The processes involved in the first few hydriding-dehydriding cycles, referred to as activation, are discussed. The controversial activation of TiFe is reviewed and new H 2-D 2 isotope scrambling results are presented which indicate that metallic iron does not play a catalytic role. The hydriding-dehydriding kinetics are resolved into three interacting processes: intrinsic kinetics, heat flow and mass flow. The intrinsic kinetics are further resolved into three processes occurring simultaneously and in series: a surface process, diffusion and phase transformation. The problem of determining the rate-limiting intrinsic process from the time-temperature-pressure ( t T P) dependences of the different processes is discussed. The employment of surface modification techniques to complement the t T P analysis is emphasized. Most kinetics analyses that have identified an intrinsic process as rate limiting are criticized as being invalid because 1. (1) the rate theory used has been inappropriate. 2. (2) heat flow has not been adequately taken into account. The viewpoint adopted is that only nucleation and growth (NG) theory is applicable. The NG formalism is developed and is employed to treat the interplay between the simultaneous intrinsic processes which is generally ignored. NG theory is also employed to model kinetics behavior in order to discuss heat flow. The control of heat flow by the previously overlooked, but very useful, control of heat capacity is discussed.