This is a brief review of the Supreme Court's Mayo decision, which is more completely reviewed in the author's article in the Journal of Tax Practice and Procedure. The essential point is that the Supreme Court has eliminated the distinction between legislative and interpretive regulations, requiring that both be given heavy deference by the courts. As a result, taxpayers will bear a heavy burden to show that an IRS regulation should be overturned. The full scope of the decision will not be known for years, since other forms of IRS guidance, e.g., Revenue Rulings and Revenue Procedures do not go through the Administrative Precedures Act's notice and comment period, which is what the Supreme Court focused on in this case.