Congenital adrenal hyperplasia (CAH) is an inherited metabolic disease caused by the deficiency of one of the enzymes necessary for cortisol synthesis. Deficiency of 21-hydroxylase (21-OH) accounts for 95% of affected patients There are two forms of the disease. The first is classic, which may be incomplete (simple virilizing) or complete (salt wasting). The second is nonclassic, which may be symptomatic or asymptomatic. In classic 21-OH deficiency which occurs in 1:10000-15000 live births, prenatal exposure to excess androgens results in virilisation of female fetus. Newborn males have normal genitalia. Postnatally, untreated females as well as males present with signs of androgen excess. Three fourths of classic 21-OH deficiency cases do not effectively synthesize aldosterone and are salt wasting, a condition that is potentially fatal. Nonclassic 21-OH deficiency, allelic variant of classic 21-OH deficiency is associated with a milder enzymatic defect and no genital ambiguity at birth, but postnatal virilization may be seen. A subset of individuals with nonclassic 21-OH deficiency are asymptomatic, despite high levels of androgens (cryptic form of disease). The 21-OH enzyme, a cytochrome P450 hemoprotein (cytochrome P450 c21) is encoded by the gene CYP21, which has a closely neighboring homologous pseudogene, CYP21P. Mutations in the CYP21 gene, causing 21-OH deficiency, are common and occur owing to two mechanisms: gene deletion and apparent gene conversion. Prenatal diagnosis is important to identify a fetus affected with 21-OH deficiency. Genital ambiguity in affected females can be prevented by proper administration of dexamethasone to pregnant mother. Postnatally, the treatment of 21-OH deficiency is lifelong hormonal replacement. With carefully supervised medical treatment. CAH patients have the capacity for normal puberty and fertility.