INTRODUCTION: Attention is drawn to the spontaneous incidence of twinning, both dizygotic and monozygotic in different mammalian species. Conjoined twinning, however, only arises when the twinning event occurs at about the primitive streak stage of development, at about 13-14 days after fertilisation in the human, and is exclusively associated with the monoamniotic monochorionic type of placentation. It is believed that the highest incidence of conjoined twinning is encountered in the human. While monozygotic twinning may be induced experimentally following exposure to a variety of agents, the mechanism of induction of spontaneous twinning in the human remains unknown. All agents that are capable of acting as a twinning stimulus are teratogenic, and probably act by interfering with the spindle apparatus. DISCUSSION: The incidence of the various types of conjoined twinning is discussed. Information from the largest study to date indicates that the spontaneous incidence is about 10.25 per million births. The most common varieties encountered were thoraco-omphalopagus (28%), thoracopagus (18.5%), omphalopagus (10%), parasitic twins (10%) and craniopagus (6%). Of these, about 40% were stillborn, and 60% liveborn, although only about 25% of those that survived to birth lived long enough to be candidates for surgery. Conjoined twinning occurs by the incomplete splitting of the embryonic axis and, with the exception of parasitic conjoined twins, all are symmetrical and "the same parts are always united to the same parts". Fusion of monozygotic twins is no longer believed to be the basis of conjoined twinning. Accounts are provided of the anatomical features of each of the commonly encountered varieties.