Diesel engines have been considered as a major source in nitrogen oxide (NOx) formation worldwide. The widespread use of diesel engines in consequence of their low fuel consumption, high durability and efficiency increases NOx emissions day by day. NOx emissions from diesel engines cause unavoidable damage on environment and people health. Although so many technologies such as exhaust gas recirculation (EGR), lean burn combustion, electronic controlling fuel injection systems, etc. have been developed to control NOx emissions from diesel engines, they couldn't meet the desired reduction in NOx emissions. In any case, Selective Catalytic Reduction (SCR) as one of the most promising aftertreatment-emission control technologies is an effective solution in restriction of NOx emissions. The use of SCR systems especially in heavy-duty diesel powered vehicles has been increasing nowadays. In these systems, to use of hydrogen (H-2) as a reductant or promoter have been improved the conversion efficiency especially at low exhaust temperatures. Many researchers have been focused on the use of H-2 in SCR systems for controlling NO emissions. In this study, the applications of H-2 in SCR of NO. have been discussed. The studies on use of H-2 in SCR of NO. emissions were examined and the effects on NOx conversions were determined. Consequently, it is confirmed that H-2 is a promising and alternative reductant in SCR of NO. and it has been kept as an attracting subject for many researchers. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.