Intact reproductive development depends on steroids and other endocrine signals. Although recent discoveries have elucidated important steps of sexual differentiation, the intricate mechanisms that regulate the development of steroid-producing tissues remain elusive. In adults, complex feedback mechanisms determine the hypothalamic and pituitary regulation of steroid hormone biosynthesis. Steroidogenic factor 1 (SF-1), an orphan member of the nuclear receptor superfamily of proteins, plays a critical role in development and differentiation of the endocrine and reproductive systems. This review provides an overview of the function of SF-1, its mechanism of action, and a perspective on the interaction of SF-1 with other determinants of sexual differentiation. Whereas SF-1 is essential for the expression of numerous steroidogenic enzymes, its presence may not correlate with steroidogenic function. Moreover, diverse co-regulators modulate the influence of SF-1 on gene transcription. The significance of these interactions is discussed in the context of reproductive development and function. Other orphan or ligand-dependent nuclear proteins may share similar mechanisms.