Shareholders challenged the Mandatory Repatriation Tax, which attributed to them a share of accumulated income earned by a controlled foreign corporation. The Supreme Court upheld the tax. Congress has long taxed shareholders or partners on their share of an entity’s realized but undistributed income, and the Sixteenth Amendment permits that attribution. The Court emphasized the narrowness of its holding and did not decide whether realization is always constitutionally required or whether Congress could tax wealth or appreciation without realization. Because the tax reached realized corporate income attributed to shareholders, the judgment was affirmed.