Detailed studied on heparin, carried out within recent 3-5 years, are reviewed. Characterization of the chemical structure and physical parameters of heparin, data on conformation of the macromolecule of heparin and its biosynthesis are discussed. The proteoglycan localization in tissues and cells is also considered. Various functions of heparin are discussed on the basis of the current information. Perspectives of future research on heparin are summarized.