Enzymatic reactions involving inorganic nitrogen species provide a rich variety of systems with which to study biological chemistry. In many cases, catalysis involves redox chemistry and takes place at metal centres. Recent structures and new spectroscopic data have rapidly advanced our knowledge of nitrogen cycle enzymology, particularly in the areas of nitrogen fixation, hydroxylamine oxidation and nitrite reduction. In the case of the nitrate reductases and nitric oxide reductase, models for structure and catalysis can be designed, based on new structural information that is now available for closely related enzymes. The past two years have also seen significant progress in our understanding of the enzymology of some 'new' reactions of the nitrogen cycle, for example anaerobic ammona oxidation and heterotrophic nitrification.