In a pending Supreme Court case, Moore v. United States, the plaintiffs are challenging the constitutionality of a provision in the 2017 Tax Cuts and Jobs Act imposing a transition tax on certain undistributed foreign profits of US taxpayers. The outcome of this case may severely constrain the ability of Congress to tax unrealized income – income that taxpayers have earned but have not yet received in the form of a cash distribution. It could place several long-standing provisions of the current federal income tax at risk and cause substantial revenue losses, while mainly benefiting high-income taxpayers.