Disorders of sex development (DSD) include a heterogeneous group of heritable disorders of sex determination and differentiation, formerly termed "intersexuality". This includes chromosomal as well as monogenic disorders, which inhibit or change primarily genetic or endocrine pathways of normal sex development. However, in most patients affected, no definitive cause for the disorder can be found. Therefore, the birth of a child with ambiguous genitalia still represents an enormous challenge. For the structuring of diagnostic procedures, for decision making and also for therapeutic interventions a highly specialized team of physicians of different subspecialties and of experts for psychosocial care is needed to counsel parents and patients accordingly. This manuscript shall explain the genetic and molecular origins of DSD, the new DSD nomenclature, the consecutive classification and steps for diagnosis. New scientific topics on DSD are presented.