The primate brain undergoes developmental changes in morphology, cellular composition, connectivity, and molecular expression. During ontogeny, environmental and social factors shape the maturation of brain structure and function. Primates are especially dependent on learning to navigate their complex social world, requiring neural plasticity. Evolutionary changes in the timing of developmental events, such as brain growth rate, synaptogenesis, and myelination, might underlie differences in cognition and learning among primate species.