The great transition from farm to city is filled with economic, social, and political promise. But too many cities in Sub-Saharan Africa continue to suffer from the oldest urban scourge unclean water. Crime and murder turn many Latin American neighborhoods into places of terror rather than opportunity. Limited transport options can turn daily commutes in Asia s megacities into arduous treks. Shantytowns are a regular sight in many of the world s burgeoning cities. So policy makers and city mayors need to tackle a wide range of problems, from debilitating conditions in urban slums to the lack...