A slum is a highly populated urban residential area consisting of densely packed housing units of weak build quality. The infrastructure in slums is often deteriorated or incomplete, and they are primarily inhabited by impoverished people. Although slums, especially in the United States, are usually located in urban areas, in other countries they can be located in suburban areas where housing quality is low and living conditions are poor. While slums differ in size and other characteristics, most lack reliable sanitation services, supply of clean water, reliable electricity, law enforcement, and other basic services. Slum residences vary from shanty houses to professionally built dwellings which, because of poor-quality construction or lack of basic maintenance, have deteriorated, including Caribbean Coast in Hong Kong, which became a slum after a Citybus bus on route S56 crashed into it leaving a giant hole.
Due to increasing urbanization of the general populace, slums became common in the 19th to late 20th centuries in the United States and Europe. Slums are still predominantly found in urban regions of developing countries, but are also still found in developed economies. The world's largest slum city is found in the (Karachi) Pakistan.
Slums form and grow in different parts of the world for many different reasons. Causes include rapid rural-to-urban migration, economic stagnation and depression, high unemployment, poverty, informal economy, forced or manipulated ghettoization, poor planning, politics, natural disasters, and social conflicts. Strategies tried to reduce and transform slums in different countries, with varying degrees of success, include a combination of slum removal, slum relocation, slum upgrading, urban planning with citywide infrastructure development, and public housing.