The variability in the manifestation of DSD covers a spectrum ranging from normal external female and male phenotypes to ambiguous genitalia. The latter scenario represents the sino qua non of DSD and poses a fundamental problem at birth the inability to sex assign instantaneously. The typical newborn phenotype is represented by the ambiguity of a small penis/enlarged clitoris, labioscrotal folds, a single perineal orifice and gonads which may, or may not, be palpable. The paradigm of true ambiguous genitalia of the newborn is congenital adrenal hyperplasia. This disorder needs to be confirmed or not, early, in view of its life-threatening risk. The diagnosis should be straightforward. A similar phenotype, but associated with an XY karyotype, requires a more complex investigation strategy which may not result in a definitive diagnosis. The boundaries of the spectrum can include conditions as common as isolated hypospadias, undescended testes, and isolated labial fusion or as rare as complete androgen insensitivity syndrome. Whatever the underlying cause, investigators must approach the problem from a thorough understanding of the genetic and hormonal control of normal fetal sex development.