Abstract: The Court's decision in Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc. presupposes that when Congress leaves indeterminacy in an organic act, that indeterminacy reflects an implicit delegation of power to the agency to fill in the details of statutory meaning. Accordingly, a reviewing court must accept the agency's interpretation if reasonable. At its threshold the Chevron test requires the reviewing court to use the "traditional tools of statutory construction" to determine if Congress expressed a clear intention concerning the interpretive question or, by virtue of indeterminacy, left the question for agency resolution. In the era in which it decided Chevron, the Court felt free to use legislative history to help determine whether Congress had directly spoken to the question at issue in the case. In the years since Chevron, the Court's understanding of the "traditional tools" of statutory interpretation has changed. Contrary to its practice at the time of Chevron, the Court has made it flatly impermissible for interpreters to rely on legislative history in a way that contradicts the text of the statute. This Article argues that the Court's new approach to legislative history precludes the Court's use of that tool of construction to resolve indeterminacy under the Chevron doctrine. If as Chevron suggests, an administrative statute's indeterminacy presumptively reflects a legislative intention to delegate broad policy-making discretion to the responsible agency, then the reviewing court's use of legislative history to narrow that discretion contradicts the implemental design of the statute by narrowing the delegation effectuated by the text.