Nucleoside monophosphokinases (NPM kinases) catalyze the reversible transfer of the terminal phosphoryl group from a nucleoside triphosphate (from ATP in most cases) to nucleoside monophophates. In mammalian tissues, at least four distinct NPM kinases have been biochemically identified: adenylate kinase, guanylate kinase, thymidylate kinase, and pyrimidine nucleoside monophosphokinase. Guanylate kinase (EC 2.7.4.8) is a small monomeric enzyme catalyzing the reversible reaction Mg²⁺ATP + (d)GMP <-> Mg²⁺ADP + (d)GDP. The enzyme plays an essential role in the recovery of (d)GMP and thus in the biosynthesis of GTP and dGTP. Its reactions and biological function closely resembling adenylate kinases, which phosphorylate (d)AMP, are also discussed. The recent knowledge of the general properties, three-dimensional structure, and properties of binding sites in these enzymes is reported.