Most of the future growth of population in the world is expected to take place in cities of developing countries. This fact alone provides a rationale for this volume, but there are also broad policy and programmatic reasons for our interest in the demography, health and well-being of urban centers in low income countries. Cities in developing countries are the power-houses of national economic growth, not least because they contain some of the most skilled, best educated and economically productive people in their respective countries. Despite these advantages, there are major threats to the future success of such cities, including those related to governance, provision of water, sanitation and housing, as well as the emergence of stark inequalities in income, wealth, and health. These issues offer challenges to our knowledge and understanding of the processes of urbanization and economic growth, provoking comparisons with the late nineteenth and early twentieth century transitions in health, mortality, fertility and economic activity in the industrializing cities of northwest Europe and North America. Today’s high income countries are struggling with the administration and financing of their modern welfare states, while leaders in low income countries are casting around for equitable solutions that produce full employment and rising incomes without leaving large sections of the population impoverished and resentful of the success of their neighbors.