Congenital adrenal hyperplasia (CAH) due to P450c21 (21-hydroxylase deficiency) is a common autosomal recessive disorder. This disorder is due to mutations in the CYP21A2 gene which is located at chromosome 6p21. The clinical features reflect the magnitude of the loss of function mutations. Individuals with complete loss of function mutations usually present in the neonatal period. The clinical features of individuals with mild loss of function mutations are predominantly due to androgen excess rather than adrenal insufficiency leading to an ascertainment bias favoring diagnosis in females. Treatment goals include normal linear growth velocity and “on-time” puberty in affected children. For adolescent and adult women, treatment goals include regularization of menses, prevention of progression of hirsutism, and fertility. This article will review key aspects regarding pathophysiology, diagnosis, and treatment of CAH.