"Residential segregation's causes are both knowable and known," writes Richard Rothstein. According to Rothstein, those causes are "20th century federal, state, and local policies explicitly designed to separate the races." Even seasoned policymakers are convinced that the residential isolation of low-income black children is only de facto--occurring in practice, but not necessarily ordained by law; that it's "an accident of economic circumstance, personal preference, and private discrimination." It's no accident, contends Rothstein. Residential segregation is actually de jure--it's the result of racially motivated public policy. The author looks at how state-sponsored segregation has played out in various cities across the United States, particularly in Louisville, Kentucky, and Seattle, Washington, and how it's now locked in place by exclusionary zoning laws. Narrowing the achievement gap will require housing desegregation: voiding exclusionary zoning, placing low- and moderate-income housing in predominantly white suburbs, and ending federal subsidies for communities that fail to reverse policies that led to racial exclusion. But relearning our racial history should be the first step. When knowledge of that history becomes commonplace, we will conclude that racially segregated school districts and metropolitan areas not only have permission, but a constitutional obligation to integrate.