A newborn infant with ambiguous genitalia is a complex enough problem to unravel without any further clouding by confusing terms. The nomenclature ‘intersex’, ‘hermaphrodite’ and ‘pseudohermaphrodite’ is anachronistic, unhelpful, and perceived to be pejorative by some affected families. In its place, a consensus statement recommends the term ‘disorder of sex development’ (DSD), a generic definition encompassing any problem noted at birth where the genitalia are atypical in relation to the chromosomes or gonads. The karyotype is used as a prefix to define the category of DSD, replacing the arcane terminology of male or female pseudohermaphroditism (now known as XY DSD or XX DSD, respectively). The new nomenclature has spawned a simple and logical classification of the causes of DSD. In this chapter new facets of gonadal dysgenesis and novel defects in steroid biosynthesis are reviewed in relation to the DSD classification, and options for early, non-invasive fetal sexing are described. Future research to determine many causes of DSD will benefit from the use of this universal language of scientific communication.