The 16th Amendment was ratified on February 3, 1913, empowering Congress to impose an income tax.  In 1894, Congress had attempted to pass an income tax, but it was struck down in a 5-4 decision by the Supreme Court in Pollock v. Farmers’ Loan & Trust157 U.S. 429 (1895).  In 1909, another bill was introduced by Progressives in Congress to impose an income tax, but conservatives attempted to thwart a tax by proposing a Constitutional amendment.  Much to their surprise, the amendment was ratified by state after state.

The Income Tax Code is codified in Title 26 of the U. S. Code.  Increasingly complex, the Internal Revenue Service issues regulations, forms and guidance for filing income tax returns.

To read more about the 16th Amendment, see:

Ratification of the Sixteenth Amendment, by Thomas B. Ripy (also available electronically through ProQuest Congressional)