The supply of affordable and adequate housing has been overwhelmed by the demand of the millions of rural poor who have migrated to cities in hope of finding better employment, health care, and better educational opportunities. According to UN estimates, the urban population of the developing world alone will increase from 2.7 billion in the year 2011 to 5.1 billion by 2050. To accommodate the more than 2 billion new arrivals, the urban footprint of cities in the developing world is expected to double by 2030—and triple by 2050. This rapid increase in both urban population and the physical size of cities implies a pressing need for housing and land. According to Reinhard Goethert of the MIT School of Architecture, “We have 20 years to build as much urban housing as was built in the past 6,000 years.”5 This is a challenge that will require an approach that is completely unprecedented in terms of scale and speed.