Slums in cities are growing globally. With more than half of the world's populace now located in urban areas, urban slums are emerging as the place for the underprivileged to live, specifically in developing countries which fail to provide slum residents with proper hygiene facilities, food, clothing and shelter. Urban slums are regions accommodating people who lack the basic necessities to sustain a healthy and safe livelihood. The largest slum in the world is Neza-Chalco-Itza in Mexico, housing almost four million people.

According to The United Nations, the definition of a slum dweller (a person living in a slum) is someone who lacks at least three of the basic requirements  among "durable walls, a secure lease or title, adequate living space, and access to safe drinking water and toilets." Slums in urban areas are usually created by the pressures of large-scale migration into cities from rural areas, driving up living costs and making city life unaffordable for many. The rural poor usually migrate into industrial cities in search of labor and better living facilities. Although to the slum dwellers, their change in lifestyle may bring many advantages, they are usually not viewed positively by the public.

Unsanitary living conditions and lack of access to basic health care mean that slum living is often associated with high rates of diseases. However, although at first sight slums might seem unpleasant for bystanders, slum residents have their own culture with unique characteristics. As an article by Elisabeth Eaves from Forbes asserts, "to the outsider, many developing-world slums look unbearably awful, but to their residents they do function, complete with social hierarchies, commerce and a degree of home-grown government."

However, features and behavioral values differ between slums based on their locality. Slums are criticized for being defined in economical terms rather than on the grounds of skill or social function. For instance, some slums such as Dharavi in Mumbai have tremendous business activities, with various forms of production such as textile, leather and cottage industries located on streets without adequate infrastructure.

There is a sense of communal association in the slums that is unmatched in the life of citizens outside. As a result of lack of space, there is close interaction within communities, and stronger bonds between people than are found in wealthier parts of the city.

Unsanitary conditions in Dharavi, Mumbai

As slums diversify, the concept of global slums is also taking shape. As Saskia Sassen writes, "most slums are not global, just like most cities are not global. But some slums are positioning themselves as actors on global stages, often with distinct political tactics and a sort of prise de conscience."  Positioning themselves globally gives these slums a sense of prestige to represent their respective locality. With concepts such as slum tourism also emerging in many developing nations, the representation of slums has gradually transitioned into the mainstream.

While most slums are in developing nations, slums have become a worldwide concept, with slum neighborhoods emerging in some developed nations such as Spain. Many projects have been undertaken to eradicate slum development, but the target set of improving slum facilities and providing better housing infrastructure for the occupants has failed to produce the desired effect. As growth in poverty and slums are entwined, eradicating urban slums is one of the toughest challenges presently facing our societies, and demands immediate attention; yet at the same time, it is equally important to realise that there may be hidden lessons that urban planners in Europe and America can learn from slums.