Byzantine Greek historiography forms an indispensable group of written sources for Eurasian history, including the history of the Mongol Empire. The sources must be interpreted within the framework of Byzantine civilization (Greek antiquity, Christianity, and the Roman imperial tradition). One of the major difficulties in interpreting Byzantine sources derives from the archaizing character of ethnonyms: one appellation may refer to several ethnic groups, and sometimes the same ethnos is designated by different terms. Another limitation is that the Byzantines’ horizon extended primarily to the western lands of the Mongol Empire, the Golden Horde, and Ilkhanid Iran. In this chapter the four major histories of the thirteenth to fourteenth centuries (Georgios Akropolites, Georgios Pachymeres, Nikephoros Gregoras, Ioannes Kantakouzenos) are treated. Then Byzantine sources relating to the Mongol period are enumerated and briefly characterized, according to their literary genres: histories, world chronicles, local histories, poems, epistles, geographical works, state and ecclesiastical documents, encomia, and oracles.