During the American Revolution, espionage was critical to both Continental and British efforts, and those employed in cloak-and-dagger operations risked death. While the most notorious episode of spying during the war, the Benedict Arnold affair, was a failure, most intelligence operations succeeded. Spycraft was no more wholly embraced than by George Washington who relied on a vast spy network and personally designed deceptions and counterintelligence efforts. In Invisible Ink: Spycraft of the American Revolution, John A. Nagy discusses the techniques used by spies during the war. Throughout, he provides examples of the codes and ciphers employed, many of which have not been previously described. In addition, the author analyzes some of the key spy rings. Based on primary research, Invisible Ink is an important contribution to the history of conflict and technology.