In this paper, a number of technical, environmental and exergetic aspects of hydrogen and hydrogen energy systems (particularly fuel cells) and their applications are discussed from an energy point of view. In addition, exergy concept is introduced for hydrogen energy systems and exergetic aspects are discussed through two illustrative examples which show a potential usefulness of exergy in hydrogen energy systems. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.