© 2007 John Wiley & Sons Ltd. All Rights Reserved. The Ni-containing acetyl-coenzyme A decarbonylases, acetyl-coenzyme A synthetases, and carbon monoxide dehydrogenases constitute a diverse family of evolutionarily primitive enzymes. They are found in anaerobic chemoautotrophic archaea and bacteria, including, for example, methanogens and homoacetogens. In the past few years there has been remarkable progress in understanding the structure and function of these enzymes. This review summarizes this progress, emphasizing spectroscopic and catalytic properties of the two Ni-Fe-S active sites (the C-cluster and A-cluster). Current controversies are discussed, including the role of a bridging sulfide ion in the C-cluster, the localization of electrons during reduction of the C- and A-clusters, the structure of a Ni-deficient precursor form of the C-cluster, and the heterogeneity problem. The large number of related gene sequences which have been recently reported are analyzed phylogenetically, and are discussed within the context of the known physiology of the organisms in which these enzymes are found. Evidence for unorthodox 'CODH-like' enzymes is presented. Microbial and evolutionary aspects are integrated with the structural, spectroscopic and mechanistic enzymology of this family of enzymes, resulting in a fairly comprehensive treatment of these enzymes and the organisms which house them.