This document is written in preparation of future standardization and provides guidance on how injection of H2 into the gas infrastructure can impact processes from the input of gas into the on-shore transmission network up to the inlet connection of gas appliances. NOTE 1 Gas infrastructure includes gas installation pipework between the delivery point of the gas and the inlet connection to the gas appliance in buildings and on industrial sites. The assessments refer to the concentrations of 2, 5, 10, 20 and up to 100 Vol.- % hydrogen in natural gas. Furthermore, it identifies the expected revision need of the existing CEN/TC 234 standards as well as the need of further new standardisation deliverables. It examines the effects on each part of the gas infrastructure in the scope of the CEN/TC 234 Working Groups 1 to 12 inclusive, based on available studies, reports and research. Due to several limitations at different hydrogen concentrations, the impacts are specified. For some specific impact, pre-standardization research is needed. By convention, for this technical report, the injection of pure hydrogen, i. e. without trace and/or minor components is considered. Awareness is given that there is the need to consider trace and/or minor components and limits set on the gas quality on European and national level, too. The information from this report is intended to define the CEN/TC 234 work program for the coverage of H2NG in relation to the scope of the CEN/TC 234 and its WGs. NOTE 2 Progress on hydrogen will develop over time. In principle this will be reflected in the standardisation process in CEN/TC 234.