The potentially null CO2 and reduced pollutants emissions of hydrogen combustion make the research effort on hydrogen Internal Combustion Engines a strategic asset for the green transition, since it can provide on one hand, a bridging technology for low range requirement mobility (passenger vehicles), and on the other hand, it could concur as definitive solution for all the heavy duty transport sectors (heavy trucks, marine and aeronautical), where the current battery storage technologies are not yet relevant to replace the conventional fossil fuel sources. This paper consists of three main parts: the first part discusses the hydrogen adopted as energy vector, in particular the potentialities and the drawbacks of this solution; then its fuel related characteristics are presented and, finally, its adoption in internal combustion engines, both in terms of engine available solutions and combustion and pollutants formation description.