Results of the study of homocysteine metabolism and its role in physiological and pathological processes are given in this review. The participation of homocysteine in the process of methionine synthesis, transsulfuration, formation of homocysteine thiolacton and their regulation, polymorphism of homocysteine metabolism enzymes, the ways of homocysteine and thiolactone incorporation into protein molecule, sources and forms of homocysteine in the blood plasma, the role of hyperhomocysteinemia in pathogenesis of cardiovascular and other diseases have been considered. Principles of homocysteine determination in the blood plasma are described here.