Alpha-fetoprotein (AFP) is a serum glycoprotein with structural and physico-chemical properties similar to albumin. However, the exact physiological functions of AFP remain unknown; those known to date include markers to pathological conditions including neoplastic and non-neoplastic diseases, antioxidant effects, growth regulator in different cells and in cancer, immune response modulator, and carrier for fatty acids and oestrogens. This review aimed to present an overview of the different functions of AFP, particularly its role in the sexual differentiation of the hypothalamus, because its ability to bind oestrogens prevents their passage to the brain, where they inhibit the surge centre development. AFP and anti-Mullerian hormone are known to be involved in the development of freemartins, or genetically female foetuses masculinised in the presence of a male co-twin.