Contrary to the dogma taught in decades which inferred that the brain contains at birth all the cells necessary for its organisation, it is now well established that certain regions of the brain generate new neurons in adulthood. In rodents and monkeys, the olfactory bulb, the hippocampus and the associative cortex form neurons throughout life. The generation of new neurons is dependent on environmental factors such as stress, learning and exercise. In rodents neurogenesis can also occur after a lesion: focal lesions in the cortex stimulate the formation of new neurons able to connect to the right target showing the capacity of the brain to regenerate in certain situations. Neurogenesis was also revealed in the adult human hippocampus suggesting that the discoveries based on monkey studies could be transposed to the human brain. Neurogenic regions contain cells able to proliferate in vitro and to generate neurons and glia. This approach allows to study human neurogenesis in the laboratory in order to understand molecular mechanisms that control neuron survival and differentiation and to potentially produce neurons for cell transplantation studies. It appears that our brain maintains, throughout life, the capacity to generate new neurons in various brain areas, probably as a way of better adapting to environment fluctuations. This characteristic opens new perspectives for therapeutical approaches.