Congenital adrenal hyperplasia (CAH) is a family of autosomal recessive disorders caused by mutations in genes encoding the enzymes involved in one of the various steps of adrenal steroid synthesis. Steroid 21-hydroxylase deficiency (21-OHD) is responsible for over 95% of the 5 forms of CAH, and results due to enzymatic defect owing to mutation in the CYP21 gene. The disease has two major clinical presentations. The "classical" form is severe, and divided into a salt wasting (SW) and simple virilizing (SV) subgroups. In both, affected female fetuses undergo virilization of the external genitalia prenatally and present at birth with sexual ambiguity. In addition, in both sexes infants with SW CAH are at risk of life-threatening adrenal crisis without treatment. This is why it is so important to make a diagnosis and to counsel the families. The diagnosis is easy by measuring the plasma levels of 17-hydroxyprogesterone (17-OHP) in antenatal (amniotic fluid), or perinatal samples (peripheral blood). Confirmation by molecular genetic analysis is advised. The second form of 21-OHD is called "non classical" because the presentation is much less severe and the onset of clinical expression occurs long after birth, often in the peripubertal period, as non-specific symptoms of hyperandrogeny. The unambiguous diagnosis of the latter requires a simple short ACTH test, with the measurement of 17-OHP at 60 min. In both forms, the mutations on the gene CYP21 responsible for the disease are now well known and can be identified by molecular biology techniques. There is a good correlation between phenotypes and genotypes, due to variable amount of the 21-hydroxylase-enzyme activity left (null to 50-60%). SW, SV and NC forms are associated with distinct mutations or combination of mutations. Nowadays, by combining hormonal and molecular tests, it is possible to predict the clinical form of the disease in a given family in the context of a prenatal diagnosis, which can lead to a prenatal treatment. Therefore, 21-OHD genotyping also appears essential for a new approach of genetic counseling, prediction of clinical form after postnatal screening and to define the post-ACTH 17-OHP values indicating the cut-off lines between NC, heterozygote and normal subjects.