Discussions of an emerging practice of “gray zone” conflict have become increasingly common throughout the U.S. Army and the wider national security community, but the concept remains ill-defined and poorly understood. This monograph aims to contribute to the emerging dialogue about competition and rivalry in the gray zone by defining the term, comparing and contrasting it with related theories, and offering tentative hypotheses about this increasingly important form of state competition. The idea of operating gradually and somewhat covertly to remain below key thresholds of response is hardly new. Many approaches being used today—such as support for proxy forces and insurgent militias—have been employed for millennia. The monograph argues that the emergence of this more coherent and intentional form of gray zone conflict is best understood as the confluence of three factors. Understood in this context, gray zone strategies can be defined as a form of conflict that pursues political objectives through integrated campaigns; employs mostly nonmilitary or nonkinetic tools; strives to remain under key escalatory or red line thresholds to avoid outright conventional conflict; and moves gradually toward its objectives rather than seeking conclusive results in a relatively limited period of time. Having examined the scope and character of gray zone conflict, the monograph offers seven hypotheses about this emerging form of rivalry. Finally, the monograph offers recommendations for the United States and its friends and allies to deal with this challenge.