The last few years have seen an increased recognition of the importance of nickel in the catalysis by enzymes of plants and microorganisms [1-6]. This review of a rapidly developing area concentrates on the catalytic functions of nickel, in those enzymes that have been demonstrated to contain it naturally. Other enzymes, which are activated by different ions, including nickel, are not covered here, but are listed in [7]. Since the previous edition [8], the status of the enzymes has changed considerably. Carbon monoxide dehydrogenase (CODH) is now recognized as being homologous to part of the acetyl coenzyme A synthase (ACS), but since it has a different function, it is classed as a separate enzyme. A new nickel enzyme, superoxide dismutase, has been added. This brings the list of distinct types of nickel-containing enzymes to six (Table 1). Advances have been made recently, notably in the areas of molecular biology, spectroscopy, and inorganic modeling. Most important, crystallographic studies have uncovered the structures of three types of enzymes.