Nearly half a century ago, in an article spanning a mere nine pages, Charles Tiebout revolutionized the way many think about American federalism. Using the analytic tools Tiebout gave them, numerous scholars now contend that, in our mobile society, citizens’ interests would best be served by dramatically shrinking the federal government and permitting state and local governments to regulate a far greater number of important matters. In this article, I argue that citizens’ interests and the dynamics of federalism are far more complicated than the devolutionary proposals of Tiebout’s adherents suggest. I identify three reasons why a mobile—and rational—public might demand federal legislation that reflects their strongly held regulatory preferences. First, citizens who might one day want or need to move across state lines have an interest in securing federal legislation aimed at maximizing the number of jurisdictions in which they would be happy to live. Second, regardless of the likelihood of their own future mobility, citizens in a mobile society have an interest in favorably shaping the norms and regulatory preferences of their future neighbors, no matter where in the country those future neighbors currently reside. Third, citizens in a mobile society have an interest in minimizing the occasions on which they feel alienated from the national community. I argue that, as a result of these interests, citizens suffer negative externalities when jurisdictions other than their own adopt regulations that those citizens find significantly objectionable. By favoring centralization on issues they regard as important, therefore, citizens manifest an intuitive understanding of a truth long recognized by political economists: one of the best remedies for interstate externalities is federal legislation. Scholars and ordinary citizens alike thus face an exceptionally difficult task—namely, determining when states and localities should regulate in an effort to maximize the number of regulatory alternatives from which mobile citizens may choose and when the federal government should regulate in an effort to minimize the negative externalities many citizens suffer when jurisdictions other than their own adopt regulations those citizens find unacceptable.