Migration patterns have been shaping the geopolitics of the Mediterranean for centuries. New populations bring with them their customs and their skills, their languages and their religions. Many factors have an effect on the type of relationship that develops between the newcomers and the original inhabitants and on the cultural transfers that may occur in both directions. Ancient historians have investigated the impact of most of the relevant factors: power relations, cultural and socio-economic differences, and technological achievements. But the impact of immigrants in these various domains is of course also very much dependent on the size of their group as compared to that of the total population. The fundamental need for the quantification of immigrant population shares, however, is often neglected in ancient history because of the scarcity of the sources. This chapter tries to fill part of this gap by focusing on a group of immigrants that has been considered particularly important in Hellenistic history: the Greeks who migrated to Egypt in the late fourth and third centuries bc.