Nitrous oxide (N ₂ O) emissions from wastewater treatment plants vary substantially between plants, ranging from negligible to substantial (a few per cent of the total nitrogen load), probably because of different designs and operational conditions. In general, plants that achieve high levels of nitrogen removal emit less N ₂ O, indicating that no compromise is required between high water quality and lower N ₂ O emissions. N ₂ O emissions primarily occur in aerated zones/compartments/periods owing to active stripping, and ammonia-oxidizing bacteria, rather than heterotrophic denitrifiers, are the main contributors. However, the detailed mechanisms remain to be fully elucidated, despite strong evidence suggesting that both nitrifier denitrification and the chemical breakdown of intermediates of hydroxylamine oxidation are probably involved. With increased understanding of the fundamental reactions responsible for N ₂ O production in wastewater treatment systems and the conditions that stimulate their occurrence, reduction of N ₂ O emissions from wastewater treatment systems through improved plant design and operation will be achieved in the near future.