In his “Prolegomena to the Medieval History of Oriental Jewry,” Eliyahu Ashtor notes the poverty of sources documenting the demographic development of Jews in medieval Islamic lands. Although scholarly estimates draw on systematic methods in forming conjectures, they nonetheless admit of wide variation. The size of any population responds to “a complex of biological, social, and cultural factors,” including plague, socioeconomic transformations, migration, and conversion. In this chapter, I discuss population estimates for Jews in Islamic lands from the rise of Islam through the fifteenth century – critically evaluating the sources, methods, and assumptions that scholars have used to arrive at them.