Two to five percent of newborn infants present with congenital structural defects. This incidence contains mostly rare to extremely rare conditions. Aetiologically, these consist of defects secondary to genetic and environmental causes, with a third group of defects which are presumed to be due to an unfavorable combination of genetic and environmental factors. In many individual patients, however, allocation into one of these groups is difficult or even impossible. The following clinical and epidemiologic observations may help in some cases to arrive at a classification; pedigree, pregnancy history (infections, teratogens, bleeding, foetal movements etc.), clinical picture and various complementary investigations. Pathogenetically, we may classify congenital structural defects into primary defects (malformations) and secondary ones, if a primarily correctly formed structure has secondarily been deformed (deformation) or destroyed (disruption). In many cases, these clinical observations will then allow a classification into one of the groups. The importance of the determination of the aetiology and pathogenesis of structural defects for the individual family is in the subsequent implications for determination of the risk of recurrence.