A slum is a densely populated urban area which is characterized by a generally low standard of living. Slums may also be known as shantytowns, barrios, ghettos, or favelas, although some of these terms have specific cultural meanings. In the later part of the 20th century, slums exploded worldwide, becoming a cause for serious concern among humanitarian organizations, as an alarmingly high number of people live in regions which could be considered slums; in Mumbai, India, for example, an estimated 60% of the population lives in slums.

Slums can form in several ways. Classically, slums have emerged in existing neighborhoods which fall upon hard times. In some cases, these neighborhoods have been prestigious and well respected. A slum forms as homes are slowly subdivided into cramped tenement apartments, and the population becomes highly concentrated. At the same time, access to services like healthcare, fresh food, and basic sanitation may start to become restricted, creating filth and squalor.

A slum can also arise from nowhere, as is the case with many of the shantytowns found in developing nations. These slums sometimes seem to emerge overnight, compacting humanity into filthy, densely packed areas with poorly constructed and often dangerous homes. In campaigns against slums, many cities have forcibly evicted people from these shantytowns, creating a ripple effect as forcibly displaced people attempt to relocate to new regions.

Most of the people who live in slums are extremely poor, and many are treated as second class citizens by their society. Health problems tend to be very high, as a result of improper sanitation and lack of access to basic health care. Malnutrition is another serious problem in many slums, as is crime, which can make a slum very dangerous for its inhabitants.

Many people view slums as the ultimate symbol of inequality, and in some regions, slums have ended up in some very unexpected locations, sometimes neighboring the homes of the wealthy. Organizations which campaign against slums argue that no human being should be forced to live in slum conditions, and that as a basic act of humanity, cities need to provide livable low cost housing and regulate construction to eliminate the growth of slums.

Unfortunately, the solution to slums is not this simple. The world's population is rapidly growing, putting immense pressure on available resources, and as developing countries become more developed, this pressure is likely to grow. Although it is somewhat disheartening to think about, gross inequality seems to go hand in hand with growing societies.