Nonclassic adrenal hyperplasia is most commonly attributable to mutations in CYP21A2 (also termed CYP21) encoding steroid 21-hydroxylase. Partial deficiency of this enzyme causes an imbalance in cortisol synthesis with consequent adrenal androgen excess. Unlike more severe forms of congenital adrenal hyperplasia, this condition is rarely recognized in infants, but rather is a potential cause of premature adrenarche and pubarche in children, virilization in young women, and variable symptoms in young men. This article will review relevant clinical, hormonal and genetic aspects of nonclassic adrenal hyperplasia.