This chapter describes the diagnosis, pathophysiology, treatments, and diagnosis of twin reversed arterial perfusion (TRAP). Existence of TRAP requires two conditions: pump or forward flow failure in the acardiac twin and a set of arterioarterial and venovenous placental anastomoses connecting the acardiac and pump twins' circulatory systems. Ultrasonography with color Doppler is the primary method for diagnosing TRAP. The added benefits of color Doppler sonography include ability to trace fetal vessels and document reversed flow through an arterioarterial anastomosis confirming diagnosis of TRAP. MRI has also been used as an adjunct modality in the diagnosis of TRAP. Using MRI, one can determine the extent of blood flow in the umbilical cord of the acardiac, as well as evaluate the pump twin for anomalies, cardiac decompensation, and signs of chronic hypoxia such as brain ischemia. However, no single surgical technique has been found to be unequivocally superior.