In the American constitutional tradition, federalism is commonly understood as a mechanism designed to institutionalize a kind of permanent struggle between state and national power. The same American constitutional tradition also holds that courts are basically passive institutions whose mission is to apply the law impartially while avoiding inherently political power struggles. These two commonplace understandings conflict on their face. The conflict may be dissolved for federal courts by conceiving their resistance to state authority as the impartial consequence of limitations on state power imposed by the United States Constitution. This reconciliation, however, is unavailable for state courts, which, by operation of the Supremacy Clause, cannot employ national law as a force for resisting national power while simultaneously appealing to it as a legitimating source of impartial restrictions on that pover. This Article argues that the tension between these understandings need not be resolved at all for state courts simply because there is nothing wrong with state courts using what powers they have to protect popular liberty by resisting, or by helping other state officials to resist, abuses of national authority. Through their control over state constitutions, state courts are capable of influencing the manner and the forcefulness with which state governments may wage the kinds of struggles against national authority contemplated by federalism. State courts may do so by folding into their interpretation of state constitutional provisions consideration of the "federalism effects" of their rulings-that is, the impact of their constructions on the ability of the state effectively to resist abuses of national power. In so doing, state courts become "agents of federalism." The Article then considers and rejects a variety of strict constructionist objections to this approach to constitutional interpretation, discusses the conditions in which state courts might receive popular authorization to act as agents of federalism, and explores some of the ramifications for state constitutional interpretation of popular decisions concerning the authority of state courts.