During this period, millions of Americans moved to cities, especially in the Northeast and Midwest. They came seeking industrial jobs, because farms were failing, and for other reasons. Many also came in the massive wave of "new" immigrants from Eastern and Southern Europe that migrated to the United States during the period. Overall, the urban population of United States increased by more than 15 million people during the period from 1870-1900, a startling transformation for a nation that had always been predominately agricultural and rural. As a result, American cities became crowded places where the extremes of wealth and poverty that characterized the Gilded Age were most evident. The poor were often crammed into dangerous tenement housing in slums that exhibited the worst features of urbanization, including crime, disease, and pollution. New York City was especially notorious for its slums, the worst being the Five Points in the Lower East Side, made famous by the photographs of Jacob Riis. Urbanization also gave rise to a new form of mass politics, one in which cynical political machines like Tammany Hall in New York, the "Gas House Gang" in Philadelphia, and many others dominated politics by appealing to the urban poor. In the process, they siphoned off millions of dollars. 

This was the dark side of urbanization. But American cities also featured many of the modern advances of the period, including electric light, skyscrapers, mass transit, and planned public spaces like Central Park in New York. Americans in urban areas could go shopping in large department stores, attend baseball games, horse races, and boxing matches, and take part in other diversions that would be familiar to us today. Cities were also the epicenters of many modern reform movements, some of which in fact emerged in response to the inner city conditions described earlier. The Progressive movement was born in large part in Northern cities.