The brain regions that control song in zebra finches are much larger in males, who sing, than in females, who do not. Two major theories have been proposed to explain sexual differentiation of the neural song circuit. The 'mammalian' theory suggests that sex steroid secretions of the tests cause masculine development in males. The 'avian' theory suggests that ovarian secretions induce feminine patterns of development in females. Although experimental evidence provides some support for the mammalian theory, neither theory comfortably predicts the outcomes of experiments that bear on the mechanisms of sexual differentiation. In particular, it has been relatively difficult to block sex steroid synthesis and action in genetic males in a way that prevents masculine neural differentiation. Moreover, genetic females that possess large amounts of testicular tissue can have a feminine neural song circuit, suggesting that testicular secretions are not solely responsible for the masculine patterns of differentiation. The results indicate that new theories are needed to explain sexual differentiation of the song system.