Nitrous oxide (N2O) is a greenhouse gas (GHG) with a lifetime of approximately 130 years in the atmosphere. The global worming effect of N2O is about 310 times higher than that of carbon dioxide (CO2) over a period of 100 years. The wastewater treatment industry is a source of N2O emission: the average N2O emission from the municipal wastewater treatment plants (MWWTPs) is estimated at 0.5-2.3% of the total N loading (IPCC, 2006). It significantly varies in different wastewater treatment processes due to the differences in wastewater composition and operation conditions. The N2O emission is up to 95% of the influent nitrogen loading rate in laboratory-scale bioreactors and up to 15% in full-scale reactors. This presentation will review the mechanisms of N2O generation in wastewater treatment facilities, the factors influencing N2O generation and some results we have obtained in our own research.