Nickel is now well recognized as an essential ultra trace element for bacteria and plants, where five distinct types of Ni-containing enzymes have been identified. It is also considered to be essential for animals and humans, however, its role in animal biochemistry is not well defined. The article, containing 10 figures and 201 bibliographic positions, provides a current summary of the properties of known nickel enzymes, with emphasis on structure-function relations. In the beginning the paper reviews occurrence of Ni in environment, and its biological importance. The relevant chemistry of nickel complexes has been shortly reviewed. Nickel enzymes are particulary prominent in the metabolism of anaerobic bacteria. For example, the metabolism of methanogens involves methyl-CoM reductase, nickel hydrogenase, acetyl-CoA synthase, and carbon monoxide dehydrogenase. Important enzyme for many bacteria, fungi, and plants is urease. Urease catalyzes the hydrolysis of urea, to from ammonia and carbamate. It was the first enzyme ever to be crystallized (Summer J.B., J. Biol. Chem., 1926, 69, 435; ibid., 1926, 70, 97). In 1975 Dixon and co-workers discovered that urease contains nickel at the active site (Dixon N.F. et.al., J. Am. Chem. Soc. 1975, 97, 4131). Twenty years later the X-ray crystal structure of the urease from Klebsiella aerogenes has been determined (Jabri E. Et al., Science, 1995, 268, 998). The enzyme is an (abg)3 trimer with each a-subunit having an (ab)8-barrel domain containing a binickel active site. A carbamylated lysine provides an oxygen ligand to each nickel, explaining why carbon dioxide is required for the activation of urease apoenzyme (Park I.S., Hausinger R.P., Science, 1995 , 267, 1156). In the paper the coordination geometry of nickel ions and the structure of active site, together with possible catalytic mechanism, are presented. [NiFe]-hydrogenases catalyze the two electron redox chemistry of H2. Crystallographic data on the hydrogenase from Desulfovibrio gigas were presented (Volbeda et al., J. Am. Chem. Soc., 1996, 118, 12989), giving new information on the structure and mode of action of its H2 activating place. The active centre was found to contain a heterodinuclear active site composed of a Ni centre bridged to an Fe centre by cysteinate ligands, and by oxygenspecies, which is proposed to be signature of the inactive from of the enzyme. The iron atom binds three diatomic ligands which are nonexchangeable triply bonded molecules (probably CO, CN- or NO). Based on the new structure possible modes of hydrogen binding and catalytic action of the active site are discussed. Methyl-coenzyme M reductase (MCR) catalyzes the final stage of the reduction of carbon dioxide to methane in methanogenic bacteria. The terminal step involves prosthetic group, Factor 430 (F-430), which in the resting state is a nickel (II) tetrapyrrole. Studies of F-430 in the enzyme complex suggest a hexacoordinate, octahedral Ni(II) environment, with two oxygen axial ligands. The spectral data of the active from of MCR are characteristic for F-430 in the Ni(I) oxidation state (Goubeaud M. Et al., Eur. J. Biochem. 1997, 243, 110), indicating that methyl-CoM reductase is activated when the enzyme-bound coenzyme F-430 is reduced to the Ni(I) state. The macrocycle can readily accommodate the structural changes that accompany reduction Ni(II) to Ni(I). Carbon monoxide dehydrogenase (CODH) catalyzes the reversible oxidation of CO to CO2, at an active site, called the cluster C, composed of an [4Fe-4S] cube with pentacoordinate Fe (called FCII), linked to a Ni(Hu Z. Et al., J. Am. Chem. Soc., 1996, 118, 830). Besides the enzymes that contain only CODH activity, there are the bifunctional enzymes that contain both CODH and acetyl-CoA synthase (ACS) activity (Ragsdale S. W., Kumar M., Chem. Rev., 1996, 96, 2515). This enzyme catalyzes reaction of CO at two separate Ni-FeS clusters. Oxidation of CO to CO2 is catalyzed by cluster C, while incorporation of CO into acetyl-CoA occurs at cluster A. A model of catalysis is proposed.