Congenital adrenal hyperplasia (CAH) comprises a group of autosomal recessive disorders caused by complete or partial defects in one of the several steroidogenic enzymes involved in the synthesis of cortisol from cholesterol in the adrenal glands. More than 95-99% of all cases of CAH are caused by deficiency of steroid 21-hydroxylase, an enzyme encoded by the CYP21A2 gene. Currently, CYP21A2 genotyping is considered a valuable complement to biochemical investigations in the diagnosis of 21-hydroxylase deficiency. More than 200 mutations have been described in literature reports, and much energy is still focused on the clinical classification of new variants. In this review, we focus on molecular genetic features of 21-hydroxylase deficiency, performing an extensive survey of all clinical pathogenic variants modifying the whole sequence of the CYP21A2 gene. Our aim is to offer a very useful tool for clinical and genetic specialists in order to ease clinical diagnosis and genetic counseling.