In 1789, the First Congress passed the “Act to establish the Treasury Department,” creating one of the three “great departments” that would form the foundation of the US federal government as we know it. Critical to this statute was the fact that Congress, in addition to the President, provided oversight of the Treasury Secretary. Specifically, Section 2 of the Act required the Treasury Secretary to “make report, and give information to either branch of the legislature, in person or in writing (as he may be required), respecting all matters referred to him by the Senate or House of Representatives, or which shall appertain to his office.” By a resolution of either house, Congress could demand the Treasury Secretary provide it with any information related to his department. This requirement did not stem from Congress’s generalized investigative, or “inquiry,” power. It was instead understood to be an exercise of a different (and apparently broader) Congressional power to order Executive Branch officials to provide information to Congress. The Treasury Act is not an aberration of American legal history, used briefly in the 1790s and then forgotten. Instead, for nearly 150 years after its passage — until at least the 1940s — the Treasury Act’s reporting requirement was widely cited as a foundational separation of powers precedent that demarcated the relative power of Congress relative to the President. That the Treasury Secretary (and, by analogy other high Executive officials) was duty-bound to report to the legislature was treated for over a century as one of our nation’s oldest separation of powers principles, dating back to the early colonial constitutions. Presidents, legislators, Supreme Court justices, and Secretaries testified to the Constitutionality and importance of the Treasury Secretary’s duty to Congress. Then, around the middle of the 20th Century, all references to the Treasury Act and its central place at the heart of separation of powers doctrine disappeared. This happened even though Section 2 of the Treasury Act in fact remains the law, codified under 31 U.S.C. § 331(d), which states “The [Treasury] Secretary shall report to either House of Congress in person or in writing, as required, on matters referred to the Secretary by that House of Congress.” Why is the Treasury Act, with its important consequences for the relationship between the President and Congress, completely absent from our “modern” separation of powers doctrine? The generical principles of Congressional non-delegation and anti-aggrandizement, elaborated in INS v. Chadha and Bowsher v. Synar, are seemingly at odds with the principle embodied in the Treasury Act. This article seeks to reestablish the centrality of the Treasury Act to any understanding of the powers of Congress and the Executive Branch relative to one another. It explores how the history of the Treasury Act, and the reporting requirement it encapsulates, may inform a new interpretation of separation of power doctrine, at least as it relates to the Treasury Department. More broadly, this article tries to better understand what the stark disconnect between theoretical and historical accounts of separation of powers doctrine says about modern constitutional interpretation in the courts and in scholarship today.