Engineered biological nutrient removal (BNR) processes have been identified by the Intergovernmental Panel on Climate Change (IPCC) as potential contributors to atmospheric nitrous oxide (N₂O) emissions. This is a significant concern to wastewater utilities because the greenhouse impact of nitrous oxide emissions on a mass equivalent basis is 300 times that of carbon dioxide. This study differs from other studies in that it characterizes the microbial pathways for N₂O formation in addition to measurement of emission rates from several BNR and non-BNR plants across the U.S. As the production and emission pathways are understood, operational strategies to minimize N₂O emissions appear highly likely.