Good housing is fundamental to physical and financial security, economic productivity, healthy communities, and human well-being—but the housing gap is huge and growing. Today about one-third of the urban population in the global South lives in informal settlements, where they tend to lack access to basic services such as electricity, running water, or sanitation. The global affordable housing gap is estimated at 330 million urban households, and this number is forecast to grow by more than 30 percent by 2025 to 440 million households, or 1.6 billion people. Many cities have attempted to solve the problem by encouraging or forcing residents to relocate to the urban periphery, but this approach has often created its own problems as people are cut off from social networks and access to employment opportunities. Addressing the challenge of adequate, secure, and affordable housing within and around the city is essential to enhancing equity, economic productivity, and environmental sustainability of the city. This translates to improved quality of life and greater equality of opportunity, thus producing a more dynamic and just city. Failure to sufficiently provide services such as water, transportation, solid waste collection, and sewerage facilities threatens the health of all urban citizens, especially the poor, and also reduces business activities.