The mammalian brain is a paradox of evolution. Although the advance in complexity of the human brain has exceeded the development of other organs, it has practically lost the ability to regenerate, and damage is repaired mainly by functional plasticity. This disparity is, however, not due to the lack of progenitor cells in the adult mammalian brain, but to their diminished or repressed capacity to replace neurons in most brain regions. Here, we discuss the current literature describing the processes of neurogenesis in the adult mammalian brain, and the recent advances in adult neural stem cells (aNSCs) with a focus on their identity, cell cycle and niche signals. Understanding these processes may hopefully lead to therapies in the future to reinstate self-repair of the brain from endogenous progenitors.