The cities of today are a product of the Industrial Revolution, which introduced large-scale manufacturing. The sheer size of the industry provoked existing problems of city life. Some of them, such as sanitation, utilities, and distribution, have been solved better than others such as housing and transportation. As metropolitan life incorporated more profitable and varied employment opportunities, rural populations became increasingly attracted to the big cities. By the 20th century, some nations were faced with shortages of agricultural workers.

Today's cities are often complex, with sub-cities inside them, e.g., Newark, N.J., falls inside the New York metropolis. The word megalopolis is at times used to describe the great swath of communities stretching North and South of New York City from Boston to Washington, D.C.

The modern cities of today are structured around rational city planning, the development of rapid transit to distant suburbs, and garden cities. There have been many reforms aimed at restoring community life for the rootless strangers so numerous in modern cities. Settlement houses, community centers, and other philanthropic and cooperative enterprises often provide this function.