Experimental research in non-human mammals indicates that testosterone exposure during early periods of rapid brain development has enduring influences on brain and behaviour. These influences are exerted when testosterone is higher in developing males than females, and the affected characteristics are those that differ by sex. Testosterone is higher in males than in females from about weeks 8 to 24 of human gestation and then again during early infancy, and both of these periods are times of rapid brain development. Substantial evidence suggests that testosterone prenatally influences human neurobehavioral development. Emerging evidence suggests that the early postnatal period is important too. This early postnatal period could provide a window for studying testosterone interacting with experience to shape human gender development.