Twin transfusion syndrome (TTTS) is the most common complication in monochorionic twins. The incidence is 30% of monochorionic twins. Its poor perinatal prognosis and the high risk of maternal complications make it one of the diseases of greatest concern in maternal-fetal medicine. Due to the large amount of vascular traffic between the placentas of monochorionic twins, the two fetuses are in direct blood communication. One fetus is a blood donor fetus with developmental delay, anemia, oliguria, low amniotic fluid, and hypotension; the other fetus is a blood recipient fetus with polycythemia, polyuria, excess amniotic fluid, cardiomegaly, hypertension, and edema. The donor fetus may die in utero due to fetal distress. The recipient fetus may also die due to heart failure. The diagnosis is made mainly by ultrasound. Differences in fetal head-rump diameter and posterior cervical cortex thickness can be detected as early as 14 weeks of gestation. Differences in amniotic fluid volume between the two fetuses, bladder volume differences, changes in the Doppler spectrum of blood flow, and edema gradually appear after 18 weeks of gestation. The current treatment for TTTS is mainly sequential amniotic fluid reduction, selective fetal reduction, and fetoscopic placental anastomosis with vascular laser coagulation, which can be selected according to the week of gestation of onset, severity of the disease, fetal complications, and parents’ expectations for the fetus.