This document describes a methodology to identify different types of methane emissions from the gas infrastructure and it explains, step by step, how to quantify each type of emission in a gas transmission, distribution and/or storage system and in an LNG terminal. Gas is considered any product with a high methane content that is in gaseous form inside the respective gas infrastructure (e.g. natural gas, biogas or mixtures thereof with each other or with hydrogen). NOTE – of the editors: The inclusion of methane emissions in LNG terminals in the prTS is in clarification with CEN/TC 282. A Mode 4 cooperation has been established by CEN/TC 234 for this purpose. Methane emission from utilisation, CNG/LNG fuelling stations, biomethane production and upgrading plants and LNG liquefaction and transport are not covered in this document, except if they are inside the covered asset (see Annex I on granularity). NOTE – 1: These principles can also be applied to other parts of the gas value chain. The document specifies a bottom-up method of quantification of identified methane sources. This quantification method requires splitting the gas systems into groups of assets, devices and components and indicating categories of emission that can be expected from these groups to determine the emission factors (EF) and the activity factors (AF). Finally, a general method to calculate the uncertainties associated with the quantified amounts of emitted methane is described. NOTE – 2: Part of the methods of this document are retrieved by an international research program initiated by GERG for DSO.