The Espionage Act of 1917 is a United States federal law enacted on June 15, 1917, shortly after the United States entered World War I. It has been amended numerous times over the years. It was originally found in Title 50 of the U.S. Code but is now found under Title 18. Specifically, it is 18 U.S.C. ch. 37.

Schenck v. United States, 249 U.S. 47 (1919), was a landmark decision of the U.S. Supreme Court concerning enforcement of the Espionage Act of 1917 during World War I. A unanimous Supreme Court, in an opinion by Justice Oliver Wendell Holmes Jr., concluded that Charles Schenck, who distributed flyers to draft-age men urging resistance to induction, could be convicted of an attempt to obstruct the draft, a criminal offense. The First Amendment did not protect Schenck from prosecution, even though, "in many places and in ordinary times, Schenck, in saying all that was said in the circular, would have been within his constitutional rights. But the character of every act depends upon the circumstances in which it is done." In this case, Holmes said, "the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent." Therefore, Schenck could be punished.