Nitrous oxide (N2O) is a potent greenhouse gas emitted during biological treatment of residual waters and can contribute significantly to the carbon footprint of the overall treatment, potentially offsetting energy-positive strategies. N2O production is mediated by three known biological pathways and through abiotic reactions, driven by biologically generated substances such as hydroxylamine and nitrite. The contributions of these different mechanism are determined by the environmental conditions and the resident microbial community. The newly discovered phenotypic diversity among aerobic ammonia oxidizers and the modularity of denitrifying pathway determines N2O emissions. Isotopic methods can be used to quantify N2O production pathways in water treatment systems, and mechanistic models can already predict N2O emissions, but limitations on their accuracy and precision still exist.