For the past half-century, legal and policy efforts to address unequal educational opportunity have largely focused on disparities between schools in the same district or between districts within a state. But the most substantial component of educational inequality across the nation is not disparities within states but disparities among states, a problem long neglected in constitutional law and public policy. In a companion article, Professor Liu argues that the Fourteenth Amendment obligates Congress to ensure that every child has adequate educational opportunity to achieve equal national citizenship. This Article examines the empirical and policy dimensions of the problem of interstate inequality. It analyzes disparities across states in terms of educational standards, resources, and outcomes, showing that the disparities disproportionately burden children who are poor, minority, or limited in English proficiency. Further, it demonstrates that interstate disparities in school spending have more to do with the ability of states to finance education than with their willingness to do so, highlighting the need for a robust federal role in promoting greater equality. Yet federal education policy has done little to ameliorate interstate disparities in education standards and resources; in fact, significant elements of current policy tend to reinforce rather than reduce such disparities. The Article thus urges Congress to pursue, within an existing framework of cooperative federalism, reforms that create national education standards and an expanded federal role in school finance to serve as building blocks of a national policy to guarantee all children educational adequacy for equal citizenship.