The construction of the brain during embryonic development was thought to be largely independent of its electrical activity. In this view, proliferation, migration and differentiation of neurons are driven entirely by genetic programs and activity is important only at later stages in refinement of connections. However, recent findings demonstrate that activity plays essential roles in early development of the nervous system. Activity has similar roles in the incorporation of newly born neurons in the adult nervous system, suggesting that there are general rules underlying activity-dependent development. The extensive involvement of activity makes it likely that it is required at all developmental stages as a necessary partner with genetic programs.