As enacted in 1939 and reenacted in 1954, the Internal Revenue Code (Code) included an ambiguous provision that extended the limitations period for the United States government (government) (defendant) to assess penalties against certain taxpayers who underreported gross income. In 1958, the United States Supreme Court decided Colony, Inc. v. Commissioner, 357 U.S. 28 (1958), construing the ambiguous provision in the 1939 Code to mean that the limitations period was not extended when a taxpayer underreported income from the sale of property. In 1984, the Supreme Court articulated the principle of Chevron deference, requiring courts to defer to an agency’s reasonable interpretation of statutory language. The Supreme Court clarified in National Cable & Telecommunications Association v. Brand X Internet Services, 545 U.S. 967 (2005), that the Supreme Court’s interpretation of unambiguous statutory language could not be displaced by an agency’s reasonable interpretation. In 2000, Home Concrete & Supply, LLC (Home Concrete) (plaintiff) understated its income from the sale of property. The government assessed a tax penalty against Home Concrete for the overstatement, relying on Treasury Department regulations promulgated in 2010 that interpreted the Code’s ambiguous provision as extending the limitations period when a taxpayer underreported income from the sale of property. Home Concrete brought suit against the government in federal district court, arguing that the limitations period should not have been extended. The district court granted partial summary judgment for the government. The court of appeals reversed. The Supreme Court granted certiorari to determine whether the Colony holding or the Treasury Department’s interpretation controlled the issue.