The transfer of a phosphoryl moiety from ATP to an acceptor group is an essential step, frequently the initial step, in the metabolism of many molecules. Hence, kinases represent a large and diverse class of enzymes present in organisms across the biological spectrum-microbial, plant, and animal. Although other kinases are mentioned below in a limited fashion, the particular focus in this article is upon kinases that transfer a phosphoryl group from ATP to the 6-hydroxyl of a hexose, most commonly glucose, to yield the 6-phosphoester. In IUB nomenclature these enzymes are referred to as ATP: D-hexose 6-phosphotransferases (EC 2.7. 1.1), with the common name of hexokinases. Typically, hexokinases are capable of phosphorylating a range of hexose substrates, but with marked preference for particular hexoses. The IUB also recognizes, as a separate class, ATP: D-gluc0se 6-phosphotransferase (EC 2.7. 1.2), kinases which show high specificity for glucose as a substrate, commonly referred to as glucokinases. The usefulness of this distinction is debatable since even enz~ wnes initially described as glucokinases have been found upon closer examination to be capable of phosphorylating hexoses other than glucose (Maitra 1975; Cfirdenas et al. 1984a) and are thus properly included under the EC 2.7. 1.1 classification, as emphasized by Cornish-Bowden and C~ trdenas (1991) and Iynedjian and Girard (1991). Moreover, as discussed below, extensive similarity in amino acid sequences, deduced from cloned cDNAs, clearly demonstrates that the glucokinases are appropriately included in the hexokinase family, and thus in this review. For the present purposes, we retain the now wellestablished" glucokinase" designations for these enzymes, even though in the strict IUB sense this designation is incorrect. Middleton (1990) has recently provided a highly readable and concise review of hexokinases and glucokinases from yeast and mammalian