In the near future the world will need to convert to a suitable, clean energy supply: one that will meet the demands of an increasing population while giving few environmental problems. One such possible supply is hydrogen. Hydrogen Energy System describes the present status of hydrogen as an energy supply, as well as its prospect in the years to come. It covers the transition to hydrogen-based, sustainable energy systems, the technology of hydrogen production, its storage and transport, and current and future hydrogen utilisation. Economic analyses of the hydrogen energy system, together with case studies, are also presented.