Oligodendrocytes provide CNS axons with myelin sheaths through processes of various lengths. The lipid-rich myelin insulates axons electrically, which increases conduction velocity. In addition, oligodendrocytes have trophic effects on axons. During development immature oligodendrocytes undergo controlled migration, proliferation and differentiation, influenced by various growth factors and axons. A number of genetically manipulated animal models have provided insights regarding myelination and the function of myelin components. Current research on myelin related diseases, i.e. multiple sclerosis, focus on novel strategies for remyelination through transplantation of myelinating cells or stimulation of endogenous oligodendrocytes.