The investigation of respiratory N-oxide reduction as part of a biogeochemical process sustained by prokaryotes has roots over a century ago and has laid the groundwork for microbial nitric oxide (NO) biology and recognizing that NO is of bioenergetic importance as an electron acceptor in anaerobic environments. NO is an obligatory respiratory substrate of nitrate- and nitrite-denitrifying prokaryotes that release nitrous oxide or dinitrogen as products. We witness currently a broadening of the scope of NO functionality and an increase in awareness that other heme-based NO-metabolizing systems contribute to the overall capability of the prokaryotic cell to cope with NO both in anaerobic and aerobic environments, including the pathogen-host interface. NO reduction of newly recognized physiological importance is catalyzed by the pentaheme nitrite reductase, cytochrome c', flavohemoglobin and flavorubredoxin. Respiratory NO reductases are heme-nonheme Fe proteins that can be classified either in a short-chain group, which are complexes with cytochrome c, or a long-chain group, which have a fused quinol oxidase domain. Even though NORs are not proton pumps, both reductase groups are structural homologues of heme-copper oxidases. As a unique case, the short-chain NOR of Roseobacter denitrificans acts on oxygen, based on a heme b3-CuB center. In turn, certain heme-copper oxidases have significant turnover rates with NO. NOR mechanisms have been proposed from oxidase active site chemistry. Besides being a respiratory substrate, NO is also a signaling molecule that triggers gene expression of the principal components of NO respiration by members of the Crp-Fnr superfamily of transcription regulators.