Steroid 21-hydroxylase is the enzyme involved in 90-95% of cases of congenital adrenal hyperplasia (CAH). The general feature of CAH, which results from an enzymatic block at any stage in the synthesis of cortisol, is increased serum adrenocorticotropin and thus stimulated adrenal synthesis of steroids proximal to the block. All enzyme defects causing CAH are autosomal recessive traits. The early steps in cortisol synthesis are common to adrenal and gonadal steroid pathways, thus CAH caused by dysfunction of the enzymes supporting these steps is also characterized by insufficient production of gonadal hormones, causing impaired genital development in the genetic male. The absence of or severe reduction of steroid 21-hydroxylase activity also causes prenatal hypersecretion of androgens by the adrenal cortex, which results in external genital ambiguity in genetic females. This paper discusses the classical and nonclassical forms of CAH, their aetiology, diagnosis and treatment, and also describes recent advances in prenatal diagnosis and treatment which avoids the need for corrective genital plastic surgery in infancy.