After the division of neuronal precursors, many of the newly generated cells become immature neurons, which migrate to their final destination in the nervous system, extend neurites and make appropriate connections. For most neurons these events occur in a narrow time window and, once in their definitive location, they immediately start the final stages of their differentiation program, remaining immature only for a short time. The main objective of this review is to present and discuss recent data on a peculiar population of cells in the adult brain, which retain an immature neuronal phenotype for an unusually prolonged time. We review and discuss recent evidence on the temporal and spatial origin of these cells, their distribution in rodents and other mammals, their structure and neurochemical phenotype, and their putative fate and function. The review is mainly focused on the population of immature neurons located in the layer II of certain cortical regions, but we will also describe similar populations found in other regions of the peripheral and central nervous systems.