Urbanization is the increasing concentration of populations in cities and the transformation of land use and society into metropolitan patterns of organization. It occurs for several reasons, including natural population increase and immigration due to push factors in rural areas and pull factors attracting people to cities. Current problems facing many urban areas include traffic and congestion, sewer and water pollution, the growth of slums and shantytowns, and urban sprawl. However, some cities have implemented strategies promoting sustainable development, smart growth, and conservation to address these issues.