A slum is a heavily populated urban informal settlement characterized by substandard housing and squalor. While slums differ in size and other characteristics from country to country, most lack reliable sanitation services, supply of clean water, reliable electricity, timely law enforcement and other basic services. Slum residences vary from shanty houses to professionally-built dwellings that because of poor-quality design or construction have deteriorated into slums. Slums were common in the 19th and early 20th centuries in the United States and Europe. More recently slums have been predominantly found in urban regions of developing and undeveloped parts of the world, but are also found in developed economies.