The most frequent form of congenital adrenal hyperplasia (CAH) is steroid 21-hydroxylase deficiency, accounting for more than 90% of cases. Affected patients cannot synthesize cortisol efficiently. Thus the adrenal cortex is stimulated by corticotropin (ACTH) and overproduces cortisol precursors. Some precursors are diverted to sex hormone biosynthesis, causing signs of androgen excess including ambiguous genitalia in newborn females and rapid postnatal growth in both sexes. In the most severe "salt wasting" form of CAH (~75% of severe or "classic" cases), concomitant aldosterone deficiency may lead to salt wasting with consequent failure to thrive, hypovolemia, and shock. Newborn screening minimizes delays in diagnosis, especially in males, and reduces morbidity and mortality from adrenal crises. CAH is a recessive disorder caused by mutations in the CYP21 (CYP21A2) gene, most of which arise from recombination between CYP21 and a nearby pseudogene, CYP21P (CYP21A1P). Phenotype is generally correlated with genotype. Classic CAH patients require chronic glucocorticoid treatment at the lowest dose that adequately suppresses adrenal androgens and maintains normal growth and weight gain, and most require mineralocorticoid (fludrocortisone). Transition of care of older patients to adult physicians should be planned in advance as a structured, ongoing process.