The Fatimids (10th - 12th centuries C.E) are known to have been the first Shiite caliphal dynasty and to have founded Cairo, the city that became their capital in 973 when they left Tunisia for Egypt. During their reign, the Fatimids built an effective war fleet that inflicted several defeats on Christian navies. This is the first study on the Fatimid naval force and, more generally, on the role of the sea for the Fatimids whose territories touched both the Mediterranean and the Red Sea. The documentation presented in this study demonstrates how, in the course of two centuries, this Ismaeli dynasty set up a maritime policy and developed a communication strategy in which their control of the sea helped legitimize their universalist claims against competing powers.