This chapter reviews the structural organization and bioenergetics of the four prokaryotic NO3 reductases and the eukaryotic enzyme and explores the possible mechanisms of NO3 transport. The membrane-bound NO3- reductase with the active site facing the cytoplasm is usually a three-subunit enzyme composed of NarGHI. The Mo ion of NarG is coordinated by an aspartate ligand provided by the polypeptide chain. Adjacent to the structural genes of NarGHI in many denitrifying bacteria are one or two members of genes encoding transport proteins generally known as NarK family proteins. Where respiratory NO3 reduction has been identified in Archaea, it is predicted to take place in a catalytic subunit with a twin arginine-dependent translocase (TAT) signal peptide, which may serve to export folded redox proteins across the cytoplasmic membrane. Periplasmic NO3 reductases (Nap) are also linked to quinol oxidation in respiratory electron transport chains but do not transduce the free energy in the QH2NO3 coupled into an H motive force. Bioinformatic analyses reveal that the Nap is phylogenetically widespread in proteobacteria, but detailed biochemical and spectroscopic studies have been restricted to enzymes from relatively few species. Some fungi have the capacity to reduce NO3 as part of a denitrification process and here the NO3 reductase is located in the mitochondrial membrane and is likely to emerge as being a prokaryotic pNar or nNar type.