The current understanding of the biology, biochemistry and genetics of the steroidogenic acute regulatory protein (StAR) and its deficiency state (lipoid congenital adrenal hyperplasia, lipoid CAH) involves the complex interplay of four areas of study: the acute regulation of steroidogenesis, clinical phenomena in lipoid CAH, the enzymatic conversion of cholesterol to pregnenolone in steroidogenic mitochondria, and the cell biology of StAR. Here, STAR is linked to congenital adrenal hyperplasia.