Realizing decreased CO2 emissions from the transport sector will be possible in the near future when substituting (part of) the currently used hydrocarbon-fuelled internal combustion engines (ICEs) with hydrogen-fuelled ICES. Hydrogen-fuelled ICEs have advanced to such a stage that, from the engine point of view, there are no major obstacles to doing this. The present paper indicates the advantages of hydrogen as a fuel for spark ignition (SI) internal combustion engines. It also shows how the hydrogen engine has matured. An extensive overview is given of the literature on experimental studies of abnormal combustion phenomena, mixture formation techniques, and load control strategies for hydrogen-fuelled engines. The Transport Technology research group of the Department of Flow, Heat and Combustion Mechanics at Ghent University has been working on the development and optimization of hydrogen engines for 15 years. An overview of the most important experimental results is presented with special focus on the most recent findings. The article concludes with a list of engine design features of dedicated hydrogen SI engines.