The paper examines the evolution of racial segregation in American cities from 1890 to 1990, focusing on the development and decline of ghettos. From 1890 to 1940, ghettos emerged as blacks migrated to urban areas, leading to vast neighborhoods predominantly filled with black housing. From 1940 to 1970, black migration continued, and ghettos expanded. Since 1970, segregation has declined as blacks moved into previously all-white areas, particularly in the South and West. The study finds a strong positive relationship between urban population density and segregation, suggesting that larger cities tend to be more segregated. Data on housing prices and attitudes toward integration indicate that mid-20th-century segregation was driven by collective actions by whites to exclude blacks from their neighborhoods. By 1990, decentralized racism, where whites paid more than blacks to live in predominantly white areas, had replaced centralized racism. The paper also explores three theories of segregation: the "port of entry" theory, the "centralized" or "collective action racism" theory, and the "decentralized racism" theory, using housing costs and attitudes to differentiate among them. The findings suggest that the decline in segregation since 1970 is driven by changes in white preferences and mobility, rather than legal barriers.The paper examines the evolution of racial segregation in American cities from 1890 to 1990, focusing on the development and decline of ghettos. From 1890 to 1940, ghettos emerged as blacks migrated to urban areas, leading to vast neighborhoods predominantly filled with black housing. From 1940 to 1970, black migration continued, and ghettos expanded. Since 1970, segregation has declined as blacks moved into previously all-white areas, particularly in the South and West. The study finds a strong positive relationship between urban population density and segregation, suggesting that larger cities tend to be more segregated. Data on housing prices and attitudes toward integration indicate that mid-20th-century segregation was driven by collective actions by whites to exclude blacks from their neighborhoods. By 1990, decentralized racism, where whites paid more than blacks to live in predominantly white areas, had replaced centralized racism. The paper also explores three theories of segregation: the "port of entry" theory, the "centralized" or "collective action racism" theory, and the "decentralized racism" theory, using housing costs and attitudes to differentiate among them. The findings suggest that the decline in segregation since 1970 is driven by changes in white preferences and mobility, rather than legal barriers.