The human brain encodes gender differences. This fact has been gradually expanded from the realm of the hypothalamus and sexual behavior to the whole brain with its different lobes, structures, and functions, by the influence of the constantly advancing neuroscientific and imag-ing research. However, less is known about sex differences in the brains of neonates and infants, despite their relevance to various sex-linked diseases that develop early in life. In this review, we provide a synopsis of the literature evidence on sex differences in the brains of neonates and infants at the morphological, structural, and network levels. We also briefly overview the pre-sent evidence on the sex bias in some brain disorders affecting infants and neonates.