Twins have higher rates of perinatal mortality, prematurity and its complications, low birth weight, intrauterine growth retardation, congenital anomalies, and long-term developmental morbidity. Monozygotic twins have lower birth weights and higher rates of congenital anomalies than dizygotic twins, which suggests that the etiology of these problems may be related to the monozygotic twinning process. Monochorionic twins have higher rates of perinatal mortality, intrapair birth weight discrepancies, and intrauterine growth retardation than dichorionic twins, which suggests that these complications may be related to placental vascular anastomoses. Monochorial vascular communications also can be responsible for twin transfusion syndrome, disseminated intravascular coagulation at birth and disruptive structural defects. Followup studies indicate that twins remain at a disadvantage for subsequent physical growth and intellectual achievement. The management of twins is challenging and fascinating because of the wide range of perinatal, neonatal, developmental, and parenting problems that can occur.