Through their deep involvement in the long-distance trade of eastern central Africa, the Yao were increasingly exposed to the impact of Swahili traders and their culture. During the nineteenth century the increased volume of trade, and the ever growing importance of slaves in that trade, combined to produce a marked growth in the scale of Yao political units. This paper begins by outlining the growth of Yao trade before the nineteenth century. It then considers the nature of Yao political organization and the way in which the slave trade, in particular, facilitated the rise of large territorial chiefdoms. The last section deals with related social and cultural changes, including the growth of towns and the introduction of Islam.