Twin-twin transfusion syndrome (TTTS) and twin anemia polycythemia sequence (TAPS) are serious complications of monochorionic (MC) twin gestations. Only MC twins have these complications because, in contrast to dichorionic twins, the circulatory systems of almost all MC twins have arteriovenous (AV) anastomoses in the placenta, thus forming a vascular connection between them. TTTS - TTTS is characterized by relative hypovolemia of one twin and hypervolemia of the other as a result of many or large AV anastomoses deep in the placenta. The cardinal prenatal finding is MC placentation with discordant amniotic fluid volumes (maximum vertical pocket < 2 cm in one amniotic sac and > 8 cm in the other amniotic sac). TAPS - TAPS are an atypical chronic form of TTTS caused by slow transfusion of red blood cells through a few very small (< 1 mm diameter) placental AV anastomoses, resulting in anemia of one twin and polycythemia of the co-twin. Amniotic fluid volumes are normal. The cardinal prenatal finding is MC placentation with middle cerebral artery-peak systolic velocity greater than 1.5 multiples of median (MoM) in one twin and less than 0.8 MoM in the other twin. These descriptions reflect the characteristics of each disorder in its classic, pure form. However, diagnosis can be challenging because TAPS, TTTS, and a third disorder, selective intrauterine growth restriction (sIUGR), are not necessarily mutually exclusive of each other and can present together in any combination. This topic will review screening, prevalence, pathophysiology, and diagnosis of TTTS and TAPS. Management and outcome of patients with TTTS and TAPS, including neurodevelopmental outcomes, are discussed separately