Although mammalian neurogenesis is mostly completed by the perinatal period, new neurons are continuously generated throughout adulthood in the restricted regions of the brain. Newly generated neurons are incorporated into the neural networks of both the hippocampal dentate gyrus and the olfactory bulb, and there is growing evidence that adult neurogenesis is important for various brain functions. Continuous neurogenesis is achieved by the coordinated proliferation and differentiation of adult neural stem cells. In this review, we discuss the recent findings concerning the roles of Notch signaling and Hes-family genes in adult neural stem cells. We also discuss the recent findings about the integration mode of new neurons into the existing neural circuits and the potential significance of adult neurogenesis in higher brain functions, such as spatial and olfactory memory.