This manuscript examines the increasingly important form of rivalry and statecraft that has become known as "gray zone strategies." In regions from Eastern Europe to the South China Sea, such tactics in the hands of ambitious regional powers pose a growing challenge to U.S. and allied interests. This monograph aims to provide a broad introduction to the issue to help leaders in the U.S. Army and the wider joint Department of Defense and national security community better understand this challenge. Dr. Michael Mazarr, a Senior Political Scientist at the RAND Corporation and Associate Program Director of the Army's Arroyo Center there, defines the issue, examines the most notable current cases of gray zone strategies, offers several hypotheses about the nature of this form of conflict, and suggests a number of policy responses. The monograph emphasizes that many gray zone tools and techniques have been employed for centuries.