Because different parts of the central nervous system form at different stages of development, there is not one critical period but many critical periods. Some neurons are formed around the time of closure of the neural tube: these include the motor horn cells of the spinal cord and some motor nuclei of the brain stem. Other neurons, most notably the granule cells of the cerebellum, olfactory bulb and hippocampus, are produced in great numbers after birth. This review focuses on the mouse, the species for which the most data on neurogenesis are available, and draws parallels with other species. The clinical significance of the chronology of neuron formation is discussed in the context of recent studies of experimentally-induced congenital brain damage.