Adult neurogenesis is the development of new neurons in the adult brain. In mammals, physiological adult neurogenesis appears to be limited to two neurogenic regions in the olfactory system and the hippocampus. Adult neurogenesis originates from neural stem cells that are integrated into a neurogenic niche. The canonical nonneurogenic regions contain precursor cells, too, but under physiological conditions neurogenesis is absent or at least extremely limited. Adult neurogenesis is regulated by numerous specific and nonspecific stimuli and appears to play a role in adapting the neuronal networks of hippocampus and olfactory bulb to situations of novelty and complexity. Failure of adult neurogenesis is discussed as pathogenic factor in a number of disorders inflicting the hippocampus, most notably temporal lobe epilepsy, major depression, and dementia.