Between 1870-1914, the United States experienced rapid urbanization as industrial cities grew in size and population. Millions of immigrants arrived from other countries, attracted by opportunities in the cities. During this period, cities transformed American society and culture. Large industrial cities developed new infrastructure like bridges and skyscrapers. However, urban living also brought problems like overcrowded tenement housing and the rise of political machines. Reformers worked to address social issues and improve conditions in cities. By the early 20th century, cities had become centers of industry, immigration, and middle class life as well as hubs of culture, education and entertainment.