The nuclear hormone receptor family--structurally-related transcriptional regulators that mediate the actions of steroid hormones, thyroid hormone, vitamin D, and retinoids--also includes orphan members that lack known activating ligands. One of these orphan receptors, steroidogenic factor 1 (SF-1), has recently been shown to play key roles in steroidogenic cell function within the adrenal cortex and gonads. SF-1 also contributes to reproductive function at all three levels of the hypothalamic-pituitary-gonadal axis. Key insights into these roles came from analyses of SF-1 knockout mice, which revealed adrenal and gonadal agenesis with consequent male-to-female sex reversal of their internal and external genitalia, impaired gonadotrope function, and agenesis of the ventromedial hypothalamic nucleus. This report reviews the data that have established SF-1 as a critical mediator of endocrine differentiation and function.