Although it was initially thought to be a passive structure, the oligodendrocyte and myelin unit is now considered highly dynamic with many important functions beyond its role in myelination. Inhibitors of neurite growth can be localized to oligodendrocyte and myelin, highlighting the importance of oligodendrocytes in general CNS maintenance and in neuronal regeneration following many types of CNS insults. Understanding of the genesis of oligodendrocytes and myelin is increasing and will surely lead to better strategies to treat demyelinating diseases including MS.