Urban decay (also known as urban rot, urban death or urban blight) is the sociological process by which a previously functioning city, or part of a city, falls into disrepair and decrepitude. There is no single process that leads to urban decay.

Urban decay can include the following aspects:

Since the 1970s and 1980s, urban decay has been a phenomenon associated with some Western cities, metropolitan areas, and megalopolis's, especially in North America and parts of Europe. Cities have experienced population flights to the suburbs, exurbs, commuter towns, villages, hamlets and outposts; often in the form of white flight. Another characteristic of urban decay is blight – the visual, psychological, and physical effects of living among empty lots, buildings, and condemned houses.

Urban decay is often the result of interrelated socioeconomic issues, including urban planning decisions, economic deprivation of the local populace, the construction of freeways and railroad lines that bypass or run through the area, depopulation by suburbanization of peripheral lands, real estate neighborhood redlining, and immigration restrictions.

During the Industrial Revolution, many people moved from rural areas to cities for employment in the manufacturing industry, thus causing urban populations to boom. Subsequent economic change left many cities economically vulnerable. Studies such as the Urban Task Force (DETR 1999), the Urban White Paper (DETR 2000), and a study of Scottish cities (2003) hypothesize areas suffering from industrial decline, high unemployment, poverty, and a decaying physical environment (sometimes including contaminated land and obsolete infrastructure) – prove "highly resistant to improvement".

Changes in transportation from public to private (specifically, to the private motor car) eliminated some of the public transport service advantages associated with cities, e.g., fixed-route buses and trains. In particular, at the end of World War II, many political decisions favored suburban development and encouraged suburbanization through financial incentives like government-supported FHA loans and VA mortgage aid. This allowed many veterans of World War II and their families to afford comfortable single-family housing in suburbs.

The manufacturing industry has historically been a base for the prosperity of major cities. When these industries relocate to larger, less urban environments, some cities have experienced population loss with associated urban decay, and even riots. Cutbacks on police and fire services may result, while lobbying for government-funded housing may increase. Increased city taxes encourage residents to move out. Libertarian economists argue that rent control contributes to urban blight by reducing new construction and investment in housing and discouraging maintenance.

Large French cities are often surrounded by areas of urban decay. While city centers tend to be occupied mainly by upper-class residents, cities are often surrounded by public housing developments, with many tenants being of North African origin (from Morocco, Algeria and Tunisia), and recent immigrants.