Nucleoside monophosphokinases (NMP kinases) catalyze the reversible transfer of the terminal phosphoryl group from a nucleoside triphosphate (ATP in most cases) to nucleoside monophosphates. In mammalian tissues, at least four distinct NMP kinases have been biochemically identified: adenylate kinase, guanylate kinase, thymidylate kinase and pyrimidine necleoside monophosphokinase. All factors affecting the concentration of intracellular adenine nucleotides play important roles in the cells. One of these factors is adenylate kinase (NTP:AMP phosphotransferase, where N is adenine or guanine, EC 2.7.4.3, myokinase, AK), a ubiquitous small monomeric intracellular enzyme that catalyzes the interconversion of adenosine neucleotides: Mg²⁺NTP+(d)AMP <===> Mg²⁺NDP+(d)ADP. In mammals, three major isozymes AK1, AK2 and AK3 were purified and studied in detail together with AK from E. coli and S. cerevisiae. The paper reports the recent knowledge of the general properties, three-dimensional structure and the properties of binding sites of these enzymes.