Oligodendrocytes are the myelin forming cells of the central nervous system. They are derived from a precursor cell, named O-2A. This precursor cell is bipotential, and can differentiate in vitro into either an oligodendrocyte or a certain type of astrocyte, named type 2 astrocyte, depending on the presence or absence of extracellular factors. The chronology of glial differentiation is now better understood. Some factors influencing the choice of the differentiation pathway have been described. Most of these factors seem to be secreted in vitro by an other type of astrocyte, type 1 astrocyte. The succession of events leading to myelination can now be analyzed at molecular level. These very recent results have mostly been obtained in vitro. Their significance in vivo has now to be ascertained.