Some of the most prominent folk singers of the 20th century—including Woody Guthrie, 'Sis Cunningham, Pete Seeger, Lee Hays, and Burl Ives—were also political activists with varying connections to the American Communist Party. As a consequence, the FBI and other governmental and right-wing organizations began monitoring them, intending to purge their voices from the airwaves. Aaron Leonard examines the cultural battle that ensued when left-wing artists exercising their First Amendment rights wound up in the FBI's crosshairs.