Summary: The twin reversed-arterial-perfusion (TRAP) sequence found in monozygotic twins is a consequence of primary or secondary cardiac development disruption and direct arterioarterial and venovenous placental anastomoses. Associated findings include the presence of a single umbilical artery (66%) and chromosomal abnormalities in the acardiac twin (33%). Morphological abnormalities in the acardiac twin are consistent with perfusion of tissues supplied by the common iliac and lower branches of the aorta with deoxygenated blood. The pump or donor twin may develop cardiac failure because of the anomalous perfusion circuit. Polyhydramnios is significantly associated with the presence of renal tissue in the acardiac twin. An acardiac pump twin weight ratio (>50%) is associated with the development of polyhydramnios and preterm labour. Identified high-risk factors for poor obstetrical outcome include: acardiac anceps, polyhydramnios, acardiac twin with ears, and pump twin cardiac failure. Management options include elective termination, observation (serial cardiotocography (CTG), ultrasonography and echocardiography) and selective nonsurgical interventions (indomethacin, digitalis, tocolysis). Additionally, surgical interventions (hysterotomy with selective delivery of the acardiac twin or ligation of the acardiac twin's umbilical cord), and ultrasound-guided embolization of the acardiac twin's umbilical artery with absolute alcohol, platinum coils, or thrombogenic coils have been reported. The most appropriate interventions for the varous clinical presentations of this disorder are as yet undetermined, and conservative nonintervention is often appropriate. Long-term follow-up data on surviving pump twins are lacking. It is anticipated that centres with active study protocols for these conditions will best serve patient care and clinical research needs.