C-peptide, which is formed during the biosynthesis of insulin, has long been considered as a biologically inactive substance. However in the recent years there is convincing evidence that the deficit of C-peptide in type 1 diabetes mellitus (DM) or its excess in DM2 lead to the development of disorders in the cardiovascular, nervous, excretory, and other systems of organism. It is shown than C-peptide in the physiological concentrations has anti-inflammatory, immunomodulatory and neuroprotective effects, so that it and its synthetic analogs can be widely used to treat diabetic patients and to prevent DM complications diabetes. To effectively use C-peptide in medicine it is necessary to study its structural-functional organization and the molecular mechanisms of regulatory action of C-peptide on the fundamental cellular processes. It is established that C-peptide coupled with Gi/o protein-coupled receptors of the serpentine type regulates the functional activity of many intracellular signaling pathways, which include phospholipase Cbeta, different forms of protein kinase C, phosphatidylinositol 3-kinases and mitogen-activated protein kinases, endothelial NO-synthase, Na+/K+-ATPase, a wide range of transcription factors and nuclear receptors. C-peptide controls the stability of the insulin hexamer complexes, and, thus,affects on the activity of insulin and insulin-regulated signaling pathways. The present review analyse the current state of the problem of structural-functional organization of C-peptide and its mechanism of action on the intracellular signaling pathways, as well as the prospects for the use of C-peptide in the fundamental biology and clinical medicine.