The nature and organization of segregation shifted profoundly in the United States over the course of the twentieth century. During the first two-thirds of the century, segregation was defined by the spatial separation of whites and blacks. What changed over time was the level at which this racial separation occurred, as macro-level segregation between states and counties gave way steadily to micro-level segregation between cities and neighborhoods. During the last third of the twentieth century, the United States moved toward a new regime of residential segregation characterized by moderating racial-ethnic segregation and rising class segregation, yielding a world in which the spatial organization of cities and the location of groups and people within them will increasingly be determined by an interaction of race and class and in which segregation will stem less from overt prejudice and discrimination than from political decisions about land use, such as density zoning.