While the Indian Ocean slave trade is at least 4,000 years old, there are three historical periods when this trade expanded significantly: at the turn of the common era (ca. 1st c. CE), the tenth to thirteenth centuries, and the nineteenth century. This chapter analyzes the ebb and flow of the slave trade in the western Indian Ocean and Red Sea region during the medieval millenium, beginning with an evaluation of how the expansion of Muslim societies impacted slavery. The regions discussed include the west coast of India, East Africa, Yemen and Arabia, Ethiopia, Nubia, and Egypt. The roles of urban markets and island entrepôt in the slave trade are discussed as well as the roles played by smaller polities along imperial frontiers. Large-scale wholesale slave trading was uncommon in the medieval Indian Ocean world. Instead, merchants generally trafficked in small numbers of enslaved people as part of larger mixed cargoes of luxury goods and other commodities. Finally, the chapter assesses recent genetics research that is relevant to tracing the movements of people through the regions of the medieval Indian Ocean.