This paper examines segregation in American cities from 1890 to 1990. From 1890 to 1940, ghettos were born as blacks migrated to urban areas and cities developed vast expanses filled with almost entirely black housing. From 1940 to 1970, black migration continued and the physical areas of the ghettos expanded. Since 1970, there has been a decline in segregation as blacks have moved into previously all white areas of cities and suburbs. Across all of these time periods there is a strong positive relation between urban population or density and segregation. Data on house prices and attitudes towards integration suggest that in the mid-20th century, segregation as a product of collective actions taken by whites to exclude blacks from their neighborhoods. By 1990, the legal barriers enforcing segregation had been replace by decentralized racism, where whites pay more than blacks to live in predominantly white areas.