A slum, as defined by the United Nations agency UN-HABITAT, is a run-down area of a city characterized by substandard housing, squalor, and lacking in tenure security. According to the United Nations, the percentage of urban dwellers living in slums decreased from 47 percent to 37 percent in the developing world between 1990 and 2005. However, due to rising population, and the rise especially in urban populations, the number of slum dwellers is rising. One billion people worldwide live in slums and the figure is projected to grow to 2 billion by 2030.

The term has traditionally referred to housing areas that were once relatively affluent but which deteriorated as the original dwellers moved on to newer and better parts of the city, but has come to include the vast informal settlements found in cities in the developing world.

Many shanty town dwellers vigorously oppose the description of their communities as 'slums' arguing that this results in them being pathologised and then, often, subject to threats of evictions. Many academics have vigorously criticized UN-Habitat and the World Bank arguing that their 'Cities Without Slums' Campaign has led directly to a massive increase in forced evictions.

Although their characteristics vary between geographic regions, they are usually inhabited by the very poor or socially disadvantaged. Slum buildings vary from simple shacks to permanent and well-maintained structures. Most slums lack clean water, electricity, sanitation and other basic services.

The rising phenomenon of slum tourism has western tourists paying to take guided tours of slums. This tourism niche is operating in almost all major slums around the world, including in Mumbai, Rio de Janeiro, Kibera, and Jakarta.