This century's biggest architectural challenge is taking place in the developing world. There, already overcrowded cities must absorb a constant influx of migrants fleeing the lack of economic opportunities or the armed conflicts plaguing their rural hometowns. Soon the world will house 1.5 billion slum dwellers, half of them in Asia, with the 2 billion mark scheduled to be reached by 2030.

In large settlements such as Rio de Janeiro's Rocinha, Nairobi's Kibera, or Mumbai's Dharavi, hundreds of thousands of residents coexist in the highest densities ever seen, often squatting on the land where they built their makeshift homes. Living conditions engender poor sanitation, a dire lack of public services, and gang-induced violence. Quick, cheap, and efficient infrastructure solutions for the urban poor were needed yesterday. However daunting, the challenges have been made more bearable by the past successes of municipality-initiated upgrading programs such as the recent one in Medellín, Colombia, and in Rio's Favela-Bairro, as well as the community-based Orangi Pilot Project in Karachi, Pakistan, led by architect Arif Hasan in the 1980s.