Adult neurogenesis has been questioned for many years. In the early 1900s, a dogma was established that denied new neuron formation in the adult brain. In the last century however, new discoveries have demonstrated the real existence of proliferation in the adult brain, and in the last decade, these studies led to the identification of neural stem cells in mammals. Adult neural stem cells are undifferentiated cells that are present in the adult brain and are capable of dividing and differentiating into glia and new neurons. Newly formed neurons terminally differentiate into mature neurons in the olfactory bulb and the dentate gyrus of the hippocampus. Since then, a number of new research lines have emerged whose common objective is the phenotypical and molecular characterization of brain stem cells. As a result, new therapies are successfully being applied to animal models for certain neurodegenerative diseases or stroke. At present, and in years to come, this finding extends to the adult human brain, and gives reason and hope to all the previous studies.