The mature nervous system comes into being during embryonic and post-embryonic development as a result of a series of developmental programs that include the birth of nerve cells and their supporting (glia) cells, the migration of these cells to defined locations within the developing nervous tissue, the establishment of connections of these cells to specific target areas, and selective neuronal cell death. Once established, we now know that the mature nervous system exhibits an amazing degree of plasticity, changing in response to experience and use. Information gained in studies of the factors that influence nervous system development and plasticity may provide a key toward unlocking the potential of the mature nervous system.