More than one half of the world population lives now in urban areas, and virtually all countries of the world are becoming increasingly urbanized. This is a global phenomenon that has nonetheless very different expressions across regions and development levels: richer countries and those of Latin America and the Caribbean have already a large proportion of their population residing in urban areas, whereas Africa and Asia, still mostly rural, will urbanize faster than other regions over the coming decades. These trends are changing the landscape of human settlement, with significant implications for living conditions, the environment and development in different parts of the world.

The urban population of the world has grown rapidly from 746 million in 1950 to 3.9 billion in 2014. Asia, despite its lower level of urbanization, is home to 53 per cent of the world’s urban population, followed by Europe with 14 per cent and Latin America and the Caribbean with 13 per cent. Managing urban areas has become one of the most important development challenges of the 21st century. Our success or failure in building sustainable cities will be a major factor in the success of the post-2015 UN development agenda.

Today, 54 per cent of the world’s population lives in urban areas, a proportion that is expected to increase to 66 per cent by 2050. Projections show that urbanization combined with the overall growth of the world’s population could add another 2.5 billion people to urban populations by 2050, with close to 90 percent of the increase concentrated in Asia and Africa. In 1990, there were ten “mega-cities” with 10 million inhabitants or more, which were home to 153 million people or slightly less than seven per cent of the global urban population at that time. In 2014, there are 28 mega-cities worldwide, home to 453 million people or about 12 percent of the world’s urban dwellers. Of today’s 28 mega-cities, sixteen are located in Asia, four in Latin America, three each in Africa and Europe, and two in Northern America. By 2030, the world is projected to have 41 mega-cities with 10 million inhabitants or more.

Countries with fast growing urbanization face numerous challenges in meeting the needs of their growing urban populations, including for housing, infrastructure, and basic services such as water and sanitation and health care.

Slums generally present a set of unique problems, including poor housing conditions, inadequate access to safe water and sanitation, overcrowding and insecure tenure; thus, the welfare of those living in these areas are seriously impacted.

Worldwide, 87% of the population gets its drinking water from improved sources, and the corresponding figure for developing regions is also high at 84%. Access is far greater, however, in urban areas (at 96%), while only 81% of rural populations have access to improved sources. However, these estimates do not take into consideration service quality (e.g. intermittent supply, disinfection) or affordability.

In 2011, a reported 2.5 billion people in the world did not use improved sanitation facilities. A comparison of the latest estimates from 2008 with those of 2000 indicates a deterioration in both water and sanitation coverage in urban areas. Over those eight years, in cities and towns of all sizes, the number of people without access to tap water at home or in the immediate vicinity increased by 114 million, and the number of people without access to private  sanitary toilets (basic sanitation) increased by 134 million. In both cases, this means an increase of 20% in the number of individuals living in cities who lack access to basic facilities.

Urban settlements are also the main source of point-source pollution. Urban wastewater is particularly threatening when combined with untreated industrial waste. In many fast-growing cities (small and medium-sized cities with populations of less than 500,000), wastewater infrastructure is non-existent, inadequate or outdated.