Interpretation of subtle neuroanatomic differences in clinical populations is illuminated by knowledge of normal brain development. Although total cerebral volume is surprisingly stable from 4 to 18 years of age, various subcomponents of the brain undergo marked and sex-specific developmental changes. These sexually dimorphic patterns of normal brain development may interact with genetic or environmental factors to account for the noted sex differences in age of onset, prevalence, and symptomatology in nearly all neuropsychiatric disorders of childhood onset. High variability of brain structure sizes during development calls for large sample longitudinal study designs.