Data related to genetics of congenital adrenal hyperplasia with emphasis on CYP21 gene defects are briefly outlined. Mutations of the StAR gene lead to impaired translocation of cholesterol from the outer mitochondrial membrane to the inner mitochondria, a rate limiting step in steroidogenesis in the adrenals and the gonads. The clinical picture is characterized by adrenal and gonadal insufficiency and sex reversal in XY individuals. Molecular defects of the CYP17 gene encoding 17alpha-hydroxylase can cause hypertension, impaired sexual maturation and impaired sexual differentiation in XY individuals. Molecular defects of the CYP11B1 gene lead to 11-hydroxylase deficiency, which is clinically expressed with virilization of the external genitalia of the female and precocious puberty in the male, as well as hypertension in both sexes. The HSD3beta1 and HSD3beta2 genes encode two isoenzymes (3betaHSDI and 3betaHSDII). The clinical picture results from either absence or diminished activity of type II 3betaHSD, resulting from mutations of the HSD3beta2 gene. The most frequent form of CAH (90% of all patients) is due to deletions, conversions or point mutations of the CYP21 gene, which encodes the enzyme 21-hydroxylase. There is a wide range of clinical expression primarily explained by the type of the molecular defect. The ratio of genotype to phenotype concordance varies in the different forms of the disease, the highest one being encountered in the non-classical form. Heterozygosity of CYP21 mutations may be expressed as premature pubarche.