A substantial number of new granule neurons are produced in the dentate gyrus in adulthood in a variety of mammalian species, including humans. Numerous studies have demonstrated that the production and survival of new hippocampal neurons can be enhanced or diminished by hormones and experience. Steroid hormones of the ovaries and adrenal glands have been shown to modulate the production of immature neurons by affecting the proliferation of granule cell precursors. Aversive experiences have been demonstrated to decrease the production of immature granule cells, whereas enriching experiences, including learning, have been shown to enhance the survival of new hippocampal cells. These studies indicate that adult-generated neurons represent a unique form of structural plasticity that can be regulated by the environment, and furthermore suggest that new neurons play an important role in hippocampal function.