The frontier is a crucial idea for understanding the primary achievement of Africans in peopling a continent known for its harsh environment. In particular, the notion is useful for tracing the dramatic spread of the Bantu-speaking peoples from West Africa through the equatorial forests into South and Central Africa over the past 5,000 years. Deployed by John Iliffe in his enormously influential history of the continent, the idea of the frontier is an important corrective to Eurocentric models of the African past in two senses. First, the history of borders is a short one, and mobility across territories occurred long before the colonial boundaries were imagined or established. Historians have been too quick to project the modern world of nation-states, with strict controls over movement, onto eras and regions where no such barriers existed.1 Secondly, there were also many internal colonizing frontiers within African regions, which experienced the spread of ideas and technologies as peoples moved. Iliffe reminds us that, contrary to popular opinion, Christianity (and Islam) preceded professional missionaries by over 1,000 years, reaching Africa across the Sahara in the hands of traders and migrants before it had entered much of Europe.2