Review is made of the positive features and the current limitations associated with the use of hydrogen as a spark ignition engine fuel. It is shown that hydrogen has excellent prospects to achieve very satisfactory performance in engine applications that may be superior in many aspects to those with conventional fuels. A number of design and operational changes needed to effect the full potential of hydrogen as an engine fuel is outlined. The question whether hydrogen can be manufactured abundantly and economically will remain the limiting factor to its widespread use as an S.I. engine fuel in the future. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.