Chevron v. NRDC
Chevron U.S.A. Inc. v. NRDC, 467 U.S. 837 (1984), is the leading opinion by the U.S. Supreme Court on reviewing agency interpretations of federal law. A unanimous opinion with three Justices recusing themselves from it, Chevron is the most frequently cited court opinion on substantive law in history. As of October 24, 2010—less than 25 years after Chevron was rendered—it was cited in writing more than 25,000 times.
General Rule for "Chevron Deference"
Under Chevron, courts must inquire "whether Congress has directly spoken to the precise question at issue." If the answer is "yes", then "that is the end of the matter" and "the court ... must give effect to [such] unambiguously expressed intent." Id. at 842-843. Put another way, the first inquiry is "whether the statute unambiguously forbids the Agency's interpretation." Barhart v. Walton, 535 U.S. 212, 218 (2002).
If the answer is "no", such that Congress has not spoken directly to the matter, then under Chevron "an agency's interpretation of its own regulation is entitled to deference." See also Christensen v. Harris County, 529 U.S. 576, 588 (2000); Gonzales v. Oregon, 126 S. Ct. at 914-915; Levine v. Apker, 455 F.3d 71, 80 (2d Cir. 2006) ("[W]e will not defer to an agency's interpretation that contravenes Congress' unambiguously expressed intent.") (internal quotation and citation omitted).
Refinements of the General Rule
While the above restatement of the Chevron rule is the common one, it is not entirely precise because it omits the importance of the tools of statutory construction. The precise rule is that the courts will "only defer, however, to agency interpretations of statutes that, applying the normal 'tools of statutory construction,' are ambiguous." INS v. St. Cyr, 533 U.S. 289, 320 n.45 (2001) (citing Id. at 843, n. 9; INS v. Cardoza-Fonseca, 480 U.S. at 447-448). In St. Cyr, the Court concluded that "[b]ecause a statute that is ambiguous with respect to retroactive application is construed under our precedent to be unambiguously prospective, Landgraf, 511 U.S. at 264, there is, for Chevron purposes, no ambiguity in such a statute for an agency to resolve." Id. at 320 n.45.