The Sixteenth Amendment (Amendment XVI) to the United States Constitution allows Congress to levy an income tax without apportioning it among the states on the basis of population.

assertions that the imposition of the U.S. federal income tax is illegal because the Sixteenth Amendment to the United States Constitution, which reads "The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration", was never properly ratified,Christopher S. Jackson, The Inane Gospel of Tax Protest: Resist Rendering Unto Caesar - Whatever His Demands, 32 Gonzaga Law Review 291, at 301-303 (hereinafter "Jackson, Gospel of Tax Protest").