A review is given of contemporary research on the hydrogen-fueled internal combustion engine. The emphasis is on light-to medium-duty engine research. We first describe hydrogen-engine fundamentals by examining the engine-specific properties of hydrogen and surveying the existing literature. Here it will be shown that, due to low volumetric efficiencies and frequent preignition combustion events, the power densities of premixed or port-fuel-injected hydrogen engines are diminished relative to gasoline-fueled engines. Significant progress has been made in the development of advanced hydrogen engines with improved power densities. We discuss several examples and their salient features. Finally, we consider the overall progress made and provide suggestions for future work. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.