This chapter examines the case of the first Almoravid amir, Yusuf ibn Tashfin (r. 1061–1106), a Lamtuna Sanhaja Berber, and the way that genealogy can legitimise political authority. More specifically, it considers how Yusuf ibn Tashfin claimed an Arab genealogy that drew on a textual heritage shared by Muslims in the East and the West. It begins with a discussion of the Almoravids and the role they played in the expansion and defence of Islam in the Islamic West; they controlled a territory, West Africa, where Islam was disseminated, as well as another region, al-Andalus, where it was necessary to defend the religion against the advances of the Christian kings from the north. The chapter then provides an overview of al-Andalus before looking at the origins of the Berbers, and in particular their relationship to the tribe of Himyar. It also analyses how genealogy enabled Yusuf ibn Tashfin to address his problems related to political and religious legitimacy.