From Raj Nallari  and Indira Iyer's lecture notes.
As quality urban housing is costly, the increasing numbers of urban poor start living in slums where water and sanitation facilities are inadequate and living conditions are crowded and often unhealthy. The UN estimates that the number of people living in slums passed 1 billion in 2007 and could reach 1.39 billion in 2020, although there are large variations among regions. In Asia and the Pacific, two out of five urban dwellers live in slums, compared with three out of five in Africa.
In percentage terms, sub-Saharan Africa has about 72 percent of city dwellers living in slums. Asia has by far the highest number of city dwellers living in slums - the problem is worst in South Asia, where half of the urban population is composed of slum dwellers. The figure below illustrates the share of slum population in some Asian and Pacific countries. In 2001, Afghanistan had as much as 99 per cent of the urban population living in slums while Nepal and Bangladesh also had high proportions-92 and 85 per cent, respectively, although they have had some success in containing the problem since 1990.
Share of slum population in urban areas in selected Asian and Pacific countries, 1990 and 2001
Source: Statistical Yearbook for Asia and the Pacific 2007
Understanding the scope and characteristics of slums has become a priority. The Millennium Development Goals , in particular, Goal 7, target 4 , calls for the improvement of the lives of at least 100 million slum dwellers. As of today, few countries are on track for reaching the goals through a rapid, sustained decline in slum growth rates. The UN-HABITAT (2006)  states that the countries that are farthest from reaching the slum target goals are mainly in sub-Saharan Africa where slums are growing at an annual rate of 4.53 percent over the period 1990-2001 mainly due to due to the rapid increases in migration coupled with insufficient capacity of local governments accommodate new residents. The largest and most overwhelming slum in Africa is Kibera in Nairobi, where between half a million and a million people reside. The UN-HABITAT statistics are both illustrative and alarming: in Zambia, 74 percent of urban dwellers live in slums; in Nigeria, 80 percent; in Sudan, 85.7 percent; in Tanzania, 92.1 percent; in Madagascar, 92.9 percent; and in Ethiopia, an amazing 99.4 percent.
Regional Characteristics of Slums, 2001
Source: UN-HABITAT. 2006
Part of the reason for burgeoning slums is the lack of an appropriate urban planning strategy. Currently, most governments have moved away from the ill-advised strategy of eliminating their urban slum problems through demolition. Strategies of governments have ranged from slum upgrading to slum relocation. Very few governments, however, are proactively planning at the appropriate scale to prevent future slums. Malawi, for example, has tackled sprawling slum development by proactively setting aside decent land in cities such as Lilongwe for low-income housing projects. With urbanization set to rapidly increase in the near future, it is imperative that governments tackle and plan for the influx of urban population to promote growth and ensure a higher quality of life for the urban poor.
- Fridays Academy