Today it is taken for granted that Congress’s power “to regulate . . . Commerce among the several States” includes the power to shut interstate markets down. That is why, for example, Congress is understood to have the power to ban the possession and use of marijuana, even though twenty states have expressed contrary preferences, either for the medicinal or recreational use of the drug. This Article argues that as a matter of constitutional history and theory both, this familiar assumption about congressional power is wrong. First, the Article demonstrates that the original understanding, which prevailed for over one hundred years, did not grant Congress the power to ban markets. Congress could pass “helper” statutes to facilitate state choices, and it could even ban particular goods (such as diseased cattle) “in service” of the interstate market; but it could not simply prohibit all commerce in products of which it disapproved. Second, the Article demonstrates that although this understanding changed following the 1903 Supreme Court decision in Champion v. Ames, none of the reasons supporting the change justify Congress possessing the power today. Finally, this Article examines theoretical justifications for congressional power grounded in law and economics and constitutional theory to suggest that the power “to regulate” interstate commerce should not be understood to include the power to prohibit it. The argument has implications for national bans on articles and activities such as interstate gambling, drugs, raw milk products and assault weapons.