Differentiation of the reproductive system occurs in stages, with early development of the gonads and later differentiation of the brain. The physiological mechanisms that are involved in the sequence of events during sexual differentiation have not been clearly understood. However, recent technological advances have made understanding these mechanisms much more accessible. Many of these techniques have been used to elucidate the nature of steroid-induced effects on target tissues, hormonal and neuroendocrine interactions, and the molecular basis of these processes. This review will focus on the sequence of events and mechanisms associated with sexual differentiation of endocrine, morphological, and behavioral components of reproduction in the chicken and Japanese quail.