I count myself among those who suppose that the Constitution contains no subject matter limits on the treaty power. More precisely, I believe that the original Constitution granted the President the power to make international agreements, with no particular constraints on the subjects they might touch. I reach this conclusion with a great deal of reluctance not because the case for this proposition is weak but because, as a matter of policy, I favor subject matter limits on the treaty power as a means of ensuring exclusive state authority over certain matters. Nonetheless, I have become convinced that the Constitution does not gratify my preferences. The treaty power is boundless in the sense that treaties of the United States can concern any subject, no matter how fanciful or seemingly absurd the matter might seem. Yet the treaty power is not completely without bounds. There likely are constraints on federal power that apply regardless of the sort of power (legislative, executive, judicial) being exercised. Such constraints would likewise apply to the treaty power as well. Part I canvasses possible subject matter limits on the treaty power. Part II discusses the Constitution’s text. Part III considers subject matter limits on treaties prior to the Constitution’s creation. Part IV examines how the Constitution constrains the treaty power that lacks subject matter bounds.