Since the foundational administrative law case of Chevron v. Natural Resources Defense Council (1984), courts have given significant deference to executive agency interpretations of federal law. United States v. Home Concrete & Supply tests whether there are any meaningful limits on such deference. The case involves a group of taxpayers who initiated a number of transactions designed to reduce their tax liability by allowing a financial entity they created, Home Concrete, to increase its tax basis and reduce its taxable gain from the sale of certain assets. In June 2003, the IRS ruled that the taxpayers’ use of Home Concrete in this way was improper and issued an adjustment to their tax return (requiring payment of back-taxes). Having missed the standard three-year limit for such actions, however, the IRS argued that the adjustment was timely under a tax-code provision that extends the statute of limitations to six years if the taxpayer “omits from gross income an amount properly includible therein which is in excess of 25 percent of the amount of gross income stated in the return.” Despite the Supreme Court’s having long ago held otherwise, Colony v. Commissioner of Internal Revenue (1956), the IRS argues that an overstatement of basis qualifies as an omission under that tax provision. Further, during the course of this litigation, the Treasury Department issued a new regulation “clarifying” the provision in a way that supported the IRS’s argument. The IRS now argues that this new regulation is controlling and should be retroactively applied to Home Concrete’s 1999 returns. After (mostly) winning at the district court, the IRS lost before the Fourth Circuit and asked the Supreme Court to review the case—which involves one of many similar applications of the relevant tax provisions. The Court took the case and now Cato has joined the National Federation of Independent Business on a brief supporting the taxpayers, arguing that sanctioning this sort of ad hoc rule-making would undermine the rule of law and the separation of powers. We note that “[t]he government’s position is that this regulation is due judicial deference” but the Supreme Court has “consistently held that where a statute has an unambiguous meaning, an agency’s contrary interpretation is not entitled to deference.” As Judge J. Harvie Wilkinson noted in his Fourth Circuit concurrence, “agencies are not a law unto themselves” and the government’s position in this case “seems to [be] something of an inversion of the universe and to pass the point where the beneficial application of agency expertise gives way to a lack of accountability and a risk of arbitrariness.” In deciding Chevron, the Supreme Court surely never intended to undermine the very structure of the Republic and unleash an administrative state wholly a law unto itself.