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. It was passed by Congress in 1909 in response to the 1895 Supreme Court case of Pollock v. Farmers' Loan & Trust Co. The Sixteenth Amendment was ratified by the requisite number of states on February 3, 1913, and effectively overruled the Supreme Court's ruling in Pollock.
Prior to the early 20th century, most federal revenue came from tariffs rather than taxes, although Congress had often imposed excise taxes on various goods. The Revenue Act of 1861 had introduced the first federal income tax, but that tax was repealed in 1872.