The hippocampal formation is widely studied in part because of its distinct and highly laminar organization as well as its demonstrated fundamental role in learning and memory. The dentate gyrus of the hippocampal formation is one of two recognized brain regions that continually generate new neurons in adulthood. In this chapter, we review the basic structure of hippocampal cellular components and circuitry, the properties of stem cells and their progeny in the dentate gyrus, and the known mechanisms and timing of their maturation and integration into the adult circuitry. We also address the functional implication of neurogenesis in the adult hippocampus.