During the last decades several means for hydrogen storage were considered. It is important to recall that the efficiency of storage is usually measured by two parameters: the gravimetric density, GD, namely the weight percentage of hydrogen stored of the total weight of the system (hydrogen+container), and the volumetric density, VD, that is the stored hydrogen mass per unit volume of the system. Both parameters are important, since for practical application a hydrogen storage device must be both light and compact. Thus the possible storage systems can be evaluated plotting their GD and VD in a Cartesian diagram, where the good ones occupy the upper right corner. This is done in Fig.1. Even the current Department of Energy-USA (DoE) targets are expressed in terms of VD and GD (green crosses in Fig.1): specifically, for on-board hydrogen storage systems for light-duty vehicles, the expected hydrogen gravimetric capacity should reach in 2015 the level of 5.5% and volumetric capacity 0.04 Kg/m3 , which would correspond to an usable energy per mass of 1.8kWh/Kg1.