All great historical boundaries are artificial. Few events are sufficiently cataclysmic to so destroy a society that virtually all elements of the earlier social system are swept away. Perhaps only the Spanish conquests in South America can merit such importance. Yet, although it is currently fashionable and reasonable to discuss the conventional boundaries between antiquity and the Middle Ages by using words such as ‘transition’ or ‘transformation’, avoiding the ﬁnality of ‘end’, events of the seventh and eighth centuries in the East appear sufﬁciently calamitous to justify the usage. Byzantium survived the onslaughts of Slavs, Avars, Arabs and Turks until 1453 and Christianity hung on in various communities of Egypt and Syria until the modern era, surviving even the crusades. Nevertheless, the cultural values and the economic and social system that had supported the cities of Roman and Byzantine Egypt gave way at some point between the sixth and the ninth centuries and, as can be seen from the survey below, the cities themselves declined and were sometimes virtually abandoned. The Byzantine urban system, with its roots in the earlier Roman and late Roman cities, was replaced by a different urban system, often focused on different urban centres, and structured in rather different ways.