In this chapter we describe the medieval sciences as studied, transmitted, and practiced within the different Jewish communities in Islamic lands. We have followed a rough chronological scheme, based on general demographic and cultural shifts. We begin by treating the period up until the tenth century, about which very little is known. In the second and longest section, we examine Jewish scientific activity through the tenth to the end of the twelfth century, first as reflected in direct sources (extant manuscripts preserved in European libraries and the documentary material in the Cairo Genizah) and then in indirect sources (nonscientific writing in which scientific theories appear in passing). After that we turn to the thirteenth and fourteenth centuries, and, finally, treat the fifteenth century on its own.