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 an important 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 characterizes the microbial pathways for N2O formation. As the production and emission pathways are understood, operational strategies to minimize N2O emissions appear highly likely.