The excitability and connectivity of adult-generated granule cells dictate to what extent newborn neurons participate in the hippocampal network. These functional parameters evolve as newborn cells mature and interact with the existing circuit. The progression of granule cell maturation during neonatal development appears to be reiterated in the adult, but with some caveats. New approaches to identify and track newborn neurons are revealing the timing of this process, as well as its sensitivity to activity-dependent regulation. © 2006 Wiley-Liss, Inc.