In the adult vertebrate CNS, oligodendrocytes are ubiquitously distributed throughout white matter. By contrast, in the embryonic CNS, recent studies suggest that oligodendrocyte precursors arise in a discrete ventral location. Thus, local ventral environmental signals that contribute to the overall patterning of the CNS are likely to influence the initial appearance of oligodendrocyte precursors. Oligodendrocyte precursors subsequently disperse through the CNS by mechanisms that remain elusive. While the program to differentiate to oligodendrocytes appears to be cell autonomous, the local environment may regulate the precise temporal and spatial patterning of myelination.