The increased use of biogas and biomethane urgently requires that these non-conventional energy gases can be transmitted through natural gas grids and refuelling stations. The European Standardization Organisation CEN is developing specifications for green gas which supplement the specifications for natural gas. The specifications for green gas address a range of parameters not commonly covered in natural gas, such as the contents of impurities (e.g., siloxanes, ammonia, halogenated hydrocarbons, and hydrogen chloride), dust content and particles. In a collaboration between 12 European metrology institutes and 3 university groups, robust and reliable methods are developed to support these draft specifications and to enable conformity assessment. The project also deals with issues related to density, calorific value, moisture content, and the sampling of biogas. This paper gives an overview of the state-ofthe-art in green gas testing, as well as an outlook what methods need be developed.