1.IntroductionAt the juncture of the 20th and 21st centuries it was realised that there was an urgent requirement for an alternative and sustainable energy vector as reserves of fossil fuels faded.1,2Moreover, if the adverse effects of global warming and consequent climate change is to be arrested then the utilisation of green and renewable energy sources is imperative. As Grochala and Edwards and later van den Berg and Areán note in their articles,3Jules Verne first brought these issues to the attention of the public in a work of fiction over 100 years ago: ‘…water will one day be employed as a fuel,that hydrogen and oxygen will constitute it,used singly or together,will furnish an inexhaustible source of heat and light’ and ‘water will be the coal of the future’ (“The Mysterious Island”, Jules Verne, 1874). Verne’s work was to presage with uncanny pertinence the role of hydrogen as the fuel of the future and predict the coming of the so-called ‘hydrogen economy’.An uninterrupted and secure energy supply for the developed nations and meeting the increasing energy demands of the rapidly developing nations are essential. The burgeoning need for energy coupled with the rapid depletion of fossil fuels pose serious threats for sustainable development. Before the potential for hydrogen as a future energy carrier can be realised, fundamental research including components of invention and discovery, subsequent implementation of new technology and socio-economic acceptance must occur. These are the key steps to thehydrogen energy transition.The greater hydrogen energy picture centres around theproductionandstorageof hydrogen, the two most important steps that currently represent a bottle neck to utilization of hydrogen more widely in fuel cell systems. It must be realized that unlike coal or oil, hydrogen is not naturally available. Thus, stable large-scale hydrogen production is necessary for the gradual switchover to the hydrogen economy. However a safe, efficient and economic storage medium is likely to be a crucial prerequisite before hydrogen would be globally acceptable as a fuel, particularly in mobile (automotive) applications. It should be emphasised here that while gaseous or liquid hydrogen is currently an option for prototype personal vehicles (cars) or larger commercial transport, solid state storage of hydrogen is potentially superior with regard to its storage capacity (both gravimetric and volumetric), energy efficiency and safety.2Nevertheless compact storage of hydrogen in a solid medium is the most demanding and challenging part of realising the hydrogen economy as far as mobile applications are concerned (Fig. 1).2bAlternatives for storage of 4 kg hydrogen, with volume relative to the size of a car. (Reprinted fromref. 2b, with permission from Macmillan Publishers Ltd.)Hydrogen storage is thus a key research area where considerable international effort is concentrated. Since this review is written from a materials perspective, current chemical research in hydrogen storage materials will be highlighted capturing the discoveries and developments over the past decade (1998–2009), the many opportunities that could be seized and the future pathways that could be taken. Although many of the materials classes meet the majority of the US Department of Energy (US-DoE) criteria for vehicular applications (e.g.hydrogen storage capacity; the amount of hydrogen stored per unit mass and per unit volume),4other factors such as non-reversibility, slow kinetics and sometimes thermodynamic barriers to hydrogen uptake-release limit or prevent their practical use. Holistic and systematic research towards understanding mechanism, structure and thermodynamics and their inter-relationships are crucial to innovation and materials development. This review illustrates the extent of emerging materials discovery and design and demonstrates how an improved chemical understanding of storage processes informs an evolving materials design strategy.