Nitrous oxide (N₂O) is a potent greenhouse gas as well as an ozone-depleting substance. N₂O is emitted during the biological nitrogen removal process in wastewater treatment systems (WTSs), and has significant environmental impacts. In this study, N₂O emission in WTSs was comprehensively reviewed to better understand the effects of key parameters on N₂O emission and obtain useful guidelines for N₂O mitigation strategies in WTSs. Three biological pathways leading to N₂O emission are hydroxylamine oxidation, nitrifier denitrification, and heterotrohic denitrification. Measurements at lab-, pilot- and full-scale WTSs have shown large variations in N₂O emission (0-95% of N-loaded) during wastewater treatment. In the full-scale WTSs (0-14.6% N₂O of N-loaded), the average and median values were 1.95% and 0.2% of N-loaded, respectively. Dissolved oxygen, nitrite concentrations, and chemical oxygen demand (COD)/N ratio are the most important parameters leading to N₂O emission. A variety of operational strategies have been suggested to minimize N₂O emission from WTSs. A new N₂O mitigation strategy involving the introduction of microorganisms with high N₂O reductase activity or oxygenic denitrification ability has been proposed as an alternative canonical denitrification.