NOTE:  Where it is feasible, a syllabus (headnote) will be released, as is being done in connection with this case, at the time the opinion is issued.
The syllabus constitutes no part of the opinion of the Court but has been prepared by the Reporter of Decisions for the convenience of the reader.
See United States v. Detroit Timber & Lumber Co., 200 U.S. 321, 337.

The Social Security Act authorizes payment of Title II disability insurance benefits and Title XVI Supplemental Security Income to individuals who have an “inability to engage in any substantial gainful activity by reason of any medically determinable … impairment … which has lasted or can be expected to last for a continuous period of not less than 12 months.” 42 U.S.C. § 423(d)(1)(A) (emphasis added); accord, §1382c(a)(3)(A). The Social Security Administration (Agency) denied benefits to respondent Walton, finding that his “inability” to engage in substantial gainful activity lasted only 11 months. The District Court affirmed, but the Fourth Circuit reversed, holding that the 12-month duration requirement modifies “impairment” not “inability,” that the statute leaves no doubt that no similar duration requirement relates to an “inability,” and that therefore Walton was entitled to benefits despite Agency regulations restricting them to those unable to work for 12 months. The court decided further that Walton qualified for benefits because, prior to his return to work, his “inability” would have been “expected” to last 12 months. It conceded that the Agency had made Walton’s actual return to work within 12 months of his onset date and before the Agency’s decision date determinative on this point, 20 CFR §§404.1520(b), 1592(d)(2), but found that the regulations conflicted with the statute. It noted that Walton’s work simply counted as part of a 9-month trial work period during which persons “entitled” to Title II benefits may work without loss of benefits, 42 U.S. C. §422(c).

Held: The Agency’s interpretations of the statute fall within its lawful interpretative authority. Pp. 4—13.

    (a) The Agency’s reading of the term “inability” is reasonable. The statute requires both an “inability” to engage in any substantial gainful activity and an “impairment” providing “reason” for the “inability,” adding that the “impairment” must last or be expected to last not less than 12 months. The Agency has determined in both its formal regulations and its interpretation of those regulations that the “inability” must last the same amount of time. Courts grant considerable leeway to an agency’s interpretation of its own regulations, and the Agency has properly interpreted its regulation here. Thus, this Court must decide (1) whether the statute unambiguously forbids that interpretation, and if not, (2) whether the interpretation exceeds permissible bounds. Chevron U.S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837, 843. First, the Act does not unambiguously forbid the regulation. That the statute’s 12-month phrase modifies only “impairment” shows only that the provision says nothing explicitly about the “inability’s” duration. Such silence normally creates, but does not resolve, ambiguity. Second, the Agency’s construction is permissible. It supplies a duration requirement, which the statute demands, in a way that consistently reconciles the statutory “impairment” and “inability” language. The Agency’s regulations also reflect the Agency’s own longstanding interpretation, which should be accorded particular deference, North Haven Bd. of Ed. v. Bell, 456 U.S. 512, 522, n. 12. Finally, Congress has frequently amended or reenacted the relevant provisions without change. Walton’s claim that Title II’s 5-month waiting period for entitlement protects against a claimant with a chronic, but only briefly disabling, disease shows, at most, that the Agency could have chosen other reasonable time periods. Moreover, Title XVI has no such period, yet Walton offers no explanation why its identical definitional language should be interpreted differently in a closely related context. Walton’s argument that the Agency’s interpretation should be disregarded because its formal regulations were only recently enacted is also rejected. E.g., Smiley v. Citibank (South Dakota), N. A., 517 U.S. 735, 741. And the Agency’s longstanding interpretation is not automatically deprived of the judicial deference otherwise its due because it was previously reached through means less formal than notice and comment rulemaking. Chevron, supra, at 843. Pp. 4—10.

    (b) Also consistent with the statute is the Agency’s regulation providing that “[y]ou are not entitled to a trial work period” if “you perform work … within 12 months of the onset of the impairment … and before the date of any … decision finding … you …disabled,” 20 CFR § 404.1592(d)(2) (emphasis added). The statute is ambiguous, and the regulation treats a pre-Agency-decision actual return to work as if it were determinative of the “can be expected to last” question. The statute’s complexity, the vast number of claims it engenders, and the consequent need for agency expertise and administrative experience lead the Court to read the statute as delegating to the Agency considerable authority to fill in matters of detail related to its administration. See Schweiker v. Gray Panthers, 453 U.S. 34, 43—44. The interpretation at issue is such a matter. Pp. 10—12.

    Breyer, J., delivered the opinion of the Court, Parts I and III of which were unanimous, and Part II of which was joined by Rehnquist, C. J., and Stevens, O’Connor, Kennedy, Souter, Thomas, and Ginsburg, JJ. Scalia, J., filed an opinion concurring in part and concurring in the judgment.