Regents Prep: U.S. History: Immigration & Migration:
Urbanization occurs when the population shifts from rural to urban areas. With the advent of innovative agricultural technologies and industrialization, Americans began to migrate to cities in droves during the 1800s. By 1920, over 50% of Americans lived in cities.

This movement had many negative consequences for the people living in cities:

Negative Effects of Urbanization

Corrupt political machines such as Tammany Hall in NYC took advantage of their positions in order to receive graft. These organizations also provided much needed services for immigrants and new city-dwellers in exchange for their vote.
The massive wave of population entering cities helped to spread diseases. To make matters worse, clean water and proper sanitation were virtually unheard of, making disease even more common.
Housing was constructed quickly and cheaply to provided for the great numbers of people entering cities. Many tenements were unhealthy due to lack of light or sanitation. Neighborhoods of tenement housing became slums where crime flourished.

Soon, many of these problem began to be addressed, and cities soon demonstrated positive characteristics. 

Positive Effects of Urbanization

Social reformers began to provide services for immigrants and new city dwellers, making them less susceptible to the influence of political machines. Examples abound, including Jane Addams who founded the Settlement House Movement with her Hull House in Chicago. This freed public money to be spent for the public good.
Museums, libraries, schools, parks, zoos, and other cultural centers were built in cities. These facilities provided urban citizens with cultural and educational opportunities that were unavailable in rural areas. 
Technology made urban living more practical and comfortable. Elevators and the use of steel girders allowed cities to grow into the sky. Mass transit was being provided by trolleys, street cars, and eventually subways. Public lighting made areas safer, while water and sewage systems improved the overall health of cities.

Suburban Sprawl
With the proliferation of the automobile due to Henry Ford's mass production assembly line, and a growing middle class, people began to move into suburbs. Levittown, on Long Island outside of New York City, was the first planned suburban development built in the early 1950s.

This trend continued through today and led to sprawl as Americans continue to spread out away from cities. This has caused an increase in traffic congestion, pollution, and infrastructure costs as American drive longer distances to and from work.