The use of sonography in obstetrics is widespread. Ultrasound is an invaluable tool in the diagnosis and management of twin gestations. This article reviews the many applications of ultrasonography to multiple pregnancies, including early diagnosis, determination of placentation and chorionicity, ultrasoundguided diagnostic procedures to determine fetal karyotype, evaluation of fetal growth and discordancy, Doppler sonography, as well as its importance in diagnosing abnormalities specific to twins, such as conjoined twins, twin-to-twin transfusion syndrome, the acardiac anomaly, and other complications associated with multiple gestations.