This chapter discusses the pathophysiological basis and pathology of twin reversed arterial perfusion (TRAP) sequence, briefly discussing prenatal diagnosis and treatment in relation to our understanding of the pathophysiology of the condition and largely focusing on the treatment options. Numerous pregnancy complications may develop, either due to consequences of the abnormal development of the acardiac fetus, or due to secondary effects on the pump twin. The abnormalities present in the affected acardiac twin represent a wide spectrum, ranging from the fetus comprising an amorphous mass of tissue through to a relatively well-formed fetus with trunk, arms, and legs. TRAP sequence is essentially a defect of monozygotic duplication requiring the presence of interfetal placental vascular anastomoses; it is therefore a complication affecting monochorionic multiple pregnancies. Since "pump" twin mortality and morbidity is well reported, several strategies have been described for antenatal intervention in TRAP sequence to improve the outcome.