Cities in Sub-Saharan Africa are experiencing rapid population growth. Yet their economic growth has not kept pace. Why? One factor might be low capital investment, due in part to Africa's relative poverty: Other regions have reached similar stages of urbanization at higher per capita GDP. This study, however, identifies a deeper reason: African cities are closed to the world. Compared with other developing cities, cities in Africa produce few goods and services for trade on regional and international markets To grow economically as they are growing in size, Africa's cities must open their doors to the world. They need to specialize in manufacturing, along with other regionally and globally tradable goods and services. And to attract global investment in tradables production, cities must develop scale economies, which are associated with successful urban economic development in other regions. Such scale economies can arise in Africa, and they will--if city and country leaders make concerted efforts to bring agglomeration effects to urban areas. Today, potential urban investors and entrepreneurs look at Africa and see crowded, disconnected, and costly cities. Such cities inspire low expectations for the scale of urban production and for returns on invested capital. How can these cities become economically dense--not merely crowded? How can they acquire efficient connections? And how can they draw firms and skilled workers with a more affordable, livable urban environment? From a policy standpoint, the answer must be to address the structural problems affecting African cities. Foremost among these problems are institutional and regulatory constraints that misallocate land and labor, fragment physical development, and limit productivity. As long as African cities lack functioning land markets and regulations and early, coordinated infrastructure investments, they will remain local cities: closed to regional and global markets, trapped into producing only locally traded goods and services, and limited in their economic growth.