Intrauterine growth restriction (IUGR) occurs in approximately 3-10% of singleton pregnancies, in 9.1% of all twins, and in 9.9% of monochorionic twins. Spontaneous demise of the IUGR twin may occur and may result in concomitant demise or severe neurological handicap of the other twin. Currently, monochorionic twins with selective IUGR (SIUGR) are managed expectantly. Alternatively, all adverse consequences resulting from the potential demise of the SIUGR twin could be averted by unlinking the circulations between the two fetuses. The latter can be achieved by laser photocoagulation of communicating vessels or by umbilical-cord occlusion. The purpose of this chapter is to review this important entity in obstetrics.