Ambiguous genitalia, sine qua non, defines a congenital endocrinopathy. The problem is immediately apparent at birth and prompts an urgent response in management which requires input from a multi-disciplinary team of experts. Assignment to a male or female gender is instantaneous when a baby is born. That this may not be possible in rare instances is immensely distressing to affected families. Indeed, abnormalities of the external genitalia sufficient to warrant genetic and endocrine studies occur in 1 in 4,500 births. There has been considerable progress in improved diagnosis and early management in recent decades, particularly with respect to congenital adrenal hyperplasia, the commonest cause of ambiguous genitalia of the newborn. For the purposes of this chapter, attention is focussed on the newborn with ambiguous genitalia and subsequent management in infancy and early childhood.