The enumeration of legislative powers in Article I of the U.S. Constitution implies that those powers must have limits. This familiar “enumeration principle” has deep roots in American constitutional history and has played a central role in recent federalism decisions of the U.S. Supreme Court. Courts and commentators, however, have seldom rigorously considered what follows from embracing it. The answer is by no means straightforward. The enumeration principle tells us that federal power must be subject to some limit, but it does not tell us what that limit should be. Nor does it tell us how the Constitution’s commitment to limited federal power should be balanced against its equally clear commitment to effective national government. Finally, the enumeration principle sheds no light on the difficult questions of judicial competence and capacity raised by a principle that requires judges to craft limits on federal power out of whole cloth. These difficulties may or may not be surmountable, but no rigorous attempt to implement the enumeration principle can avoid grappling with them.