Everyone agrees that federal statutes can "preempt" or displace state laws. Nearly everyone agrees, moreover, that the preemptive effects of federal statutes derive from the Supremacy Clause of the federal Constitution. Yet courts have long treated the Supremacy Clause chiefly as a symbol-a rhetorical expression of federal dominance, but a provision with little practical content of its own. They have looked elsewhere to devise concrete tests for preemption.

This Article challenges conventional assumptions about the vapidity of the Supremacy Clause. The Supremacy Clause, I argue, was drafted and understood against the backdrop of an established framework for determining when one statute repealed or displaced another. Understood in that context, the Supremacy Clause supplies a precise measure of the trigger for preemption. The Supremacy Clause tells courts that they must apply valid rules of federal law to the cases that come before them. When state law contradicts federal law (in the sense that applying a rule of state law would entail disregarding a valid rule of federal law), the Supremacy Clause operates to displace the state law. But nothing in the Supremacy Clause displaces state law in other situations. The Supremacy Clause leaves courts free to apply state law except to the extent that doing so would keep them from obeying the Clause's direction to follow all valid rules of federal law.

It follows that the Supremacy Clause does not itself support expansive formulations of "obstacle preemption," under which state law is preempted whenever its practical effects will hinder accomplishment of the full purposes and objectives behind a federal statute. To be sure, some federal statutes may themselves imply preemption clauses along these lines; as a matter of statutory interpretation, they might impliedly establish the rule that states cannot adopt or enforce laws that would get in the way of certain federal objectives. But different federal statutes pursue their objectives to the different extents, and not all federal statutes can be read to imply broad rules of "obstacle preemption." By formulating "obstacle preemption" as an across-the-board rule, equally applicable to all federal statutes, the modern Supreme Court has gone astray.

The connection between the Supremacy Clause and the traditional framework for repeals also sheds light on a puzzle about the Clause's language. Modern readers tend to assume that much of the Supremacy Clause is redundant; once we have been told that federal law is supreme and binding, we see no reason to be told that it applies "any Thing in the Constitution or Laws of any State to the Contrary notwithstanding." Legal drafters of the eighteenth and nineteenth centuries, however, would have recognized this phrase as a "non obstante clause." Such clauses were common in American session laws, and they had a well-understood meaning: they acknowledged that a statute might partially repeal some other laws, and they told courts not to apply the traditional presumption against implied repeals.

The presence of a non obstante provision in the Supremacy Clause confirms the connection between preemption and repeals. But it also undermines the artificial "presumption against preemption" that both courts and commentators have embraced. The non obstante provision tells courts that even if a particular interpretation of a federal statute would contradict (and therefore preempt) some state laws, this fact is not automatically reason to prefer a different interpretation. In the absence of other indications, courts should not struggle to adopt "narrowing" constructions of federal statutes in order to avoid preemption; courts should give federal statutes their natural meaning and let them displace whatever state law they contradict.