Neurogenesis occurs throughout adulthood in the mammalian brain by coordinated proliferation and differentiation of adult neural stem cells. Newborn neurons are incorporated into the functional networks of both the olfactory bulb and the hippocampal dentate gyrus, suggesting significant roles of adult neurogenesis in brain functions. In this review, we discuss the recent findings about the integration mode of new neurons into the existing neural circuits. We further address the potential significance of adult neurogenesis in higher brain functions such as olfactory and spatial memory.