Congenital adrenal hyperplasia describes a group of inherited autosomal recessive disorders characterized by an enzymatic defect in cortisol biosynthesis, compensatory increases in corticotropin secretion, and adrenocortical hyperplasia. 21-Hydroxylase deficiency is responsible for more than 95% of cases and is one of the most common known autosomal recessive disorders. The classic or severe type presents in the newborn period or early childhood with virilization and adrenal insufficiency, with or without salt loss; the mild or nonclassic form presents in late childhood or early adulthood with mild hyperandrogenism and is an important cause of masculinization and infertility in women. This wide range of phenotypic expression is mostly explained by genetic variation, although genotype-phenotype discrepancies have been described. Reproductive, metabolic, and other comorbid conditions, including risk for tumors, are currently under investigation in both forms of the disease. A high proportion of patients with adrenal incidentalomas may be homozygous or heterozygous for 21-hydroxylase deficiency. Women with congenital adrenal hyperplasia often develop the polycystic ovary syndrome. Ectopic adrenal rest tissue is often found in the testes of men with congenital adrenal hyperplasia; characteristic clinical and radiologic findings help differentiate this tissue from other tumors. Levels of corticotropin-releasing hormone are elevated in patients with depression and anxiety and are expected to be elevated in patients with congenital adrenal hyperplasia; it is unknown whether patients with 21-hydroxylase deficiency have an increased incidence of these psychiatric disorders. Abnormalities in both the structure and function of the adrenal medulla have been shown in patients with classic congenital adrenal hyperplasia, and the degree of adrenomedullary impairment may be a biomarker of disease severity. The 21-hydroxylase-deficient mouse has provided a useful model with which to examine disease mechanisms and test new therapeutic interventions in classic disease, including gene therapy. Treatment of this condition is intended to reduce excessive corticotropin secretion and replace both glucocorticoids and mineralocorticoids. However, clinical management is often complicated by inadequately treated hyperandrogenism, iatrogenic hypercortisolism, or both. New treatment approaches currently under investigation include combination therapy to block androgen action and inhibit estrogen production, and bilateral adrenalectomy in the most severely affected patients. Other approaches, which are in a preclinical stage of investigation, include treatment with a corticotropin-releasing hormone antagonist and gene therapy.