In the last decade, the mechanical alloying and mechanical grinding techniques have been used extensively to synthesise or modify hydrogen storage materials in an attempt to improve their hydrogen sorption properties. Since hydrogen diffusion through grain boundaries is usually much faster than in the bulk and since calculations show that the H/M ratio in metallic clusters containing hydrogen can reach high values when the size of clusters is reduced to the nanometer scale, one was hoping that ball milled nanocrystalline metal hydrides could have better absorption/desorption kinetics as well as improved storage capacity. The kinetics and the activation are improved in most cases but we usually observe a loss in capacity in as-milled AB. AB(5) and AB(2) hydrogen storage systems. For Mg-based hydrides, however, the loss of capacity is negligible. The loss of storage capacity is usually attributed to structural disorder which could be topological or chemical in nature. For instance, the loss of capacity and the change in the equilibrium plateau pressure in ball-milled nanocrystalline FeTi was attributed to amorphous grain boundary phases. Fortunately, this loss of capacity can be recovered by thermal treatment at relatively low temperature. In this paper, we report and compare the hydrogen storage properties of mechanically treated low and high temperature metal hydride systems and discuss the influence of structural disorder, microstrain and grain size on properties.