At City Journal, Cities from Scratch - A new path for development. A quote:
During the twenty-first century, more urbanization will take place than during all of human history. Both the global population and the fraction of it living in cities will reach their peaks, as 3 billion to 5 billion people join the 3 billion who already call cities home. That staggering urban growth will be strongest in the developing world. If the existing cities there simply grow by accretion, many people will end up in dangerous slums.