As cities and towns in developing countries grow at unprecedented rates and set the social, political, cultural and environmental trends of the world, one of the most pressing challenges for the global community in the twenty-first century is sustainable urbanization. In 1950, one-third of the world’s population lived in cities. Just 50 years later, this proportion had risen to one-half and will continue to grow to two-thirds, or six billion people, by 2050. Cities are the world’s economic, cultural and social powerhouses, yet they also create many challenges, including population pressures and poverty. In many cities, especially those in developing countries, slum dwellers account for more than 50 per cent of the population and have little or no access to adequate shelter, water and sanitation.