Cite as: 533 U. S. 289 (2001)
Opinion of the Court
"The inquiry into whether a statute operates retroactively demands a commonsense, functional judgment about 'whether the new provision attaches new legal consequences to events completed before its enactment.' " Martin, 527 U. S., at 357-358 (quoting Landgraf, 511 U. S., at 270). A statute has retroactive effect when it " 'takes away or impairs vested rights acquired under existing laws, or creates a new obligation, imposes a new duty, or attaches a new disability, in respect to transactions or considerations already past . . . .' " 46 Id., at 269 (quoting Society for Propagation of the Gospel v. Wheeler, 22 F. Cas. 756, 767 (No. 13,156) (CC NH 1814) (Story, J.)). As we have repeatedly counseled, the judgment whether a particular statute acts retroactively "should be informed and guided by 'familiar considerations of fair notice, reasonable reliance, and settled expectations.' " Martin, 527 U. S., at 358 (quoting Land-graf, 511 U. S., at 270).
IIRIRA's elimination of any possibility of § 212(c) relief for people who entered into plea agreements with the expectation that they would be eligible for such relief clearly " 'attaches a new disability, in respect to transactions or considerations already past.' " Id., at 269. Plea agreements involve a quid pro quo between a criminal defendant and the government. See Newton v. Rumery, 480 U. S. 386,
portation proceedings initiated after IIRIRA's effective date. We only defer, however, to agency interpretations of statutes that, applying the normal "tools of statutory construction," are ambiguous. Id., at 843, n. 9; INS v. Cardoza-Fonseca, 480 U. S., at 447-448. Because 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.
46 As we noted in Hughes Aircraft Co. v. United States ex rel. Schumer, 520 U. S. 939 (1997), this language by Justice Story "does not purport to define the outer limit of impermissible retroactivity." Id., at 947. Instead, it simply describes several "sufficient," as opposed to "necessary," conditions for finding retroactivity. Ibid.
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