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In this Article, part of a symposium on federal preemption of state tort law, I build upon my earlier work on the nature of preemption to try and deepen the conceptual and constitutional foundations of the subject. I argue that this neglected dimension must be moved to center stage if preemption doctrine is to have a coherent and principled framework. In particular, the key issues are the nature, source, and limits of Congress's power to preempt the states. The result is that preemption should be understood as a discretionary power of Congress the source of which lies in the Necessary and Proper Clause. Most importantly, one and the same constitutional limit on this power is strongly suggested by analyzing it from all four of the following perspectives: (a) its source in the Necessary and Proper Clause; (b) its similarity to two other congressional powers to alter federal-state relations, (c) the political safeguards of federalism, and (d) comparative constitutional law. This constitutional limit is that although Congress has the power to preempt the states, it can only do so expressly. There should be no such thing as implied preemption. In the final section, I illustrate the practical workings of my recast preemption doctrine by applying it to the leading cases of recent years.