Cleveland Walton (plaintiff) applied for disability benefits and Supplemental Security Income under a provision of the Social Security Act (Act) that authorizes disability benefits for individuals unable to engage in any substantial gainful activity by reason of a medically determinable mental impairment that has lasted or can be expected to last for a continuous period of not less than twelve months. Upon reviewing his application, the Social Security Administration (SSA) (defendant) determined that Walton had developed a serious mental illness that caused him to lose his job as a full-time teacher. The agency further found that Walton was able to engage in substantial gainful employment as a cashier eleven months after losing his teaching job. The SSA denied Walton’s claim on the ground that his inability to engage in substantial gainful employment lasted eleven, not twelve, months. In doing so, the agency interpreted the Act to mean that a claimant is not disabled if, within twelve months after its onset, the impairment no longer prevents substantial gainful employment. Walton challenged the agency’s decision in federal court. The district court affirmed the agency’s decision, but the court of appeals reversed on the ground that the Act’s twelve-month duration requirement modified the word “impairment,” not the word “inability.” The United States Supreme Court granted certiorari.