Ammonia oxidizing bacteria (AOB) and archaea (AOA) derive their energy for life from nitrification: the proton-coupled multi-electron oxidation of ammonia.1 This bioenergetic lifestyle presents numerous challenges unique from those underlying primary metabolism of sugars. For example, many intermediates in nitrification are gases, many are cytotoxic, and many undergo multiple competing redox reactions––to the point that some metabolites are explosive. Thus, the study of nitrification by AOB and AOA promises many insights into how nature achieves tight control over chemical outcomes. This lecture will survey the past decade of research by the Lancaster group into the metalloenzyme catalyzed steps of nitrification by both AOB and AOA. Key results include the discovery of nitric oxide as an obligate intermediate of nitrification by AOB,2 the means of achieving selectivity and control over iron-catalyzed hydroxylamine oxidation,3 and the recent discovery of a multicopper oxidase AOA that generates nitroxyl that in turn produces dinitrogen and nitrous oxide.4