Twin reversed arterial perfusion sequence (TRAP) is a rare complication of monochorionic twins consisting of complete retrograde blood perfusion with consequent developmental disruption and malformation of one twin (acardiac twin), blood bypass of the placenta and subsequent hemodynamic overload in the otherwise normally developed twin (pump twin). Recent studies have identified a higher incidence than historically reported and identification in early pregnancy is crucial to establish opportune diagnosis and management. If not detected, the survival rate of the pump twin is only 50 – 60%, as heart failure, fetal hydrops and intrauterine death may develop, as well as miscarriage or preterm delivery may occur. As a result of increasing awareness and early diagnosis, treatment options have shifted from the second to the first trimester in recent years, although optimal timing and mode of intervention is still controversial. In this chapter the incidence, pathophysiology and clinical presentation of TRAP sequence will be discussed, as well as different therapeutic options will be addressed.