During the past few years, significant advances have been made in meeting structure of myelin sheaths and the mechanisms participating in myelination and demyelination. Myelin is formed by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS). Myelin is composed of several layers of membranes wrapped around axons. In contrast majority of biological membranes both CNS and PNS myelin is characterized by a high ratio between lipids and proteins. Myelin in CNS and PNS don't differ in respect to lipids, but their protein components are different. Better understanding of myelin structure and function and as well as processes of myelination and demyelination will help to clarify some aspects of demyelinating diseases and injures in the nervous system.