Postnatal brain development is the result of an increase in structural complexity, rather than an increase in volume. The course of neural development can be traced through four overlapping processes: cell birth (neurogenesis), migration, formation of connectivity (including elaboration of processes, synapse formation, cell death, and axonal regression), and myelination. In mammals, there are important species differences in the duration of each of these processes as well as their timing with respect to birth. In all mammalian species, however, the basic pattern of neural development involves exuberant production of neural elements followed by widespread elimination of neurons or regression of some of their axon collaterals. Neuronal connections increase during brain maturation. Neurons appear at about 10 to 18 post-menstrual weeks, neuroglia from 15 postmenstrua! week to 2 postnatal years. Myelinisation starts after cell proliferation and migration, and continues to adulthood.