The scope of Congress’s authority under the Necessary and Proper Clause is being challenged by a theory that is gaining acceptance in the courts and in legal scholarship. The “great powers” theory posits that some implied powers, even if necessary to effectuate an enumerated power, are not “proper” because of the degree of their importance. According to its advocates, powers that are great, important and substantive cannot derived from implication. This theory is said to enforce the principle that implied powers are necessarily inferior to express powers and to explain why some seemingly incidental powers, but not others, are listed as enumerated powers in Article I, Section 8. This theory is gaining traction. It has recently been adopted by three Supreme Court Justices (including by the Chief Justice in the first Health Care Case) and defended in important scholarly works. Critics have challenged this theory as being too indeterminate to apply and contradicted by the conventional reading of McCulloch. The criticisms in this article are more fundamental — the great powers theory is unsound historically, …