Since 1750, the year that commonly marks the start of the Industrial Revolution, the atmospheric concentrations of carbon dioxide, methane and nitrous oxide have risen about 40 %, 150 % and 20 %, respectively, above the pre-industrial levels due to human activity (IPCC (2013) Climate Change 2013: The Physical Science Basis. Working Group I Contribution to the IPCC 5th Assessment Report, IPCC, Cambridge, United Kingdom and New York, NY, USA). These elevated greenhouse gas concentrations are held responsible for climate change, which has detrimental effects on the global ecosystem. The treatment of municipal wastewater entails the emission of greenhouse gases. The emission of carbon dioxide and the extent to which it contributes to the increased atmospheric greenhouse gas concentrations is well understood. The origins of methane and nitrous oxide, both potent greenhouse gases with a global warming potential of respectively 34 and 298 CO2 equivalents over a 100 year time horizon, are far less understood. This lack of insight hampers the mitigation of these emissions. The present thesis discusses the emission of nitrous oxide and methane from municipal wastewater treatment plants. The final goal is to come up with guidelines to mitigate these emissions in order to decrease the climate footprint of wastewater treatment. This requires insight into the extent of the emissions and into the relationships between the emissions on the one hand and the plant’s operational conditions on the other hand. This work fulfils the need for decent emission data by providing long-term, online emission data from a covered wastewater treatment plant that resulted in the most precise and accurate emission estimate from a full-scale plant to date. Given the importance of reliable data, particular attention is paid to sampling techniques (dissolved methane) and sampling strategies (nitrous oxide).