Midkine is a multifunctional peptide which, together with pleiotrophin, forms a structurally distinct family of heparin-binding growth factors. The paper presents the structure of midkine together with its amino-acid sequence and the functions of its domains as well as structural differences between the physiological forms of this peptide and those found in tumors. The localization of the midkine gene and its tissue expression both in embryonic life and in mature organisms is described. The available information on midkine receptors is discussed in detail. Most often they seem to be proteoglycans containing heparan sulfate and chondroitin sulfate, which can also be a part of the multimolecular receptor complexes binding midkine. The variety of midkine receptors is probably responsible for the diverse biological activity of this peptide. The paper presents up-to-date views on the biological activity of midkine both at the cellular (mitogenic properties, participation in apoptosis, and cellular migration, adhesion, morphological differentiation, and chemokine synthesis stimulation) and tissue levels (participation in organogenesis, tissue regeneration and protection and in the formation and degradation of extracellular matrix).