The existing European Union (EU) natural gas network provides large capacity to inte-grate renewable (RGs) and low-carbon gases. Today, hydrogen contributes only a few percentto Europe’s energy consumption and is almost exclusively produced from fossil fuels and usedin the industry. Nevertheless, hydrogen has a significant role to play in emission reduction inhard-to-decarbonize sectors, in particular, as a fuel in transport applications and as a fuel orfeedstock in certain industrial processes (steel, refining or chemical industries, the productionof “green fertilizers”). Carbon dioxide (CO2) in reaction with hydrogen can also be furtherprocessed into synthetic fuels, such as synthetic kerosene in aviation. In addition, hydrogenbrings other environmental co-benefits when used as fuel, such as the lack of air pollutantemissions.However, in transitional phase from fossil to RG, namely, renewable or green hydrogen,natural gas/biomethane and hydrogen blends, are needed to gradually replace natural in exist-ing gas transmission and distribution networks. The gas networks are believed to be able touse natural gas/biomethane and hydrogen blends with 5–20 % of hydrogen by volume. Mostsystems and applications are able to handle it without a need for major infrastructure upgradesor end-use appliance retrofits or replacements. The promotion of hydrogen network such as European Hydrogen backbone (EHB) is gaining momentum in Europe. To decarbonize thenatural gas grids, the threshold of hydrogen in the existing grid systems must be increased,which can be done by means of wider natural gas/biomethane and hydrogen blending andsimultaneous transportation in currently operational gas networks.