Twin-to-twin transfusion syndrome (TTTS) is a complication unique to monochorionic (MC) twins and affects 10–15% of MC twins, including MC twin pairs in higher order multiple pregnancies. The disease has fascinated obstetric specialists since it was first described in 1875 , when it was recognized as a condition not amenable to treatment and with a very high perinatal loss rate. Since then, TTTS has provided many challenges, not only in terms of deciphering the underlying pathogenesis but also in attempting to alter the clinical course of a condition with a >90% perinatal loss rate untreated and very high (>50%) neurological morbidity in any surviving babies [2, 3]. Outcomes improve dramatically with treatment, which will be discussed in detail in Chapters 32, 33, and 35. However, even with treatment 12% of TTTS pregnancies will end with a double fetal loss, and only 62% with two survivors .