Pauline Thomas (plaintiff) was employed as an elevator operator. In August 1995, Thomas’s job was eliminated. In June 1996, Thomas applied for disability-insurance benefits under the Social Security Act (Act), 42 U.S.C. § 301, et. seq., based on Thomas’s heart disease and cervical and lumbar radiculopathy. The Social Security Administration (SSA) (defendant) denied Thomas’s application. In a hearing, an administrative law judge (ALJ) concluded that Thomas was not under a disability because she was still able to perform her previous work as an elevator operator. After the SSA’s Appeals Council denied her request for review, Thomas challenged the ALJ’s determination in district court on the ground that she was unable to perform her previous work because the job no longer existed in significant numbers in the national economy. The district court affirmed. The United States Court of Appeals for the Third Circuit reversed in favor of Thomas. The United States Supreme Court granted the SSA’s petition for certiorari.