In a world where there is a growing awareness of the possible effects of human activities on climate change, there is a need to identify the emission of greenhouse gases (GHG) from wastewater treatment plants (WWTPs). As a result of this growing awareness, governments started to implement regulations that require water authorities to report their GHG emissions. With these developments there exists a strong need for adequate insight into the emissions of N2O and CH4. With this insight water authorities would be able to estimate and finally reduce their emissions. The overall objectives of the different research programs performed by partners of the GWRC members WERF (United States of America), WSAA (Australia), CIRSEE-Suez (France) and STOWA (the Netherlands) were: To define the origin of N2O emission. To understand the formation processes of N2O. To identify the level of CH4 emissions from wastewater collection and treatment systems. To evaluate the use of generic emission factors to estimate the emission of N2O from individual plants