This chapter focuses on the book's primary written sources on the Mongols, including the work of Sunnī Muslim authors, Shīʻī Muslims, and eastern Christians who wrote under Mongol domination. After a brief overview on the Mongolian and other Far Eastern material, the chapter discusses nine categories of sources, most of them in Persian or Arabic. Among them are the works of Muslim observers contemporary with the early Mongol invasions, Muslim historians writing under the pagan Ilkhans, Muslim authors active in Iran following the conversion of the Ilkhans, and Christians from Latin Europe. The chapter considers why these authors told the story in the manner they did, their preoccupations and guiding purposes, and their intended readers.