Scholars have long recognized and debated the effects of the “security dilemma,” where efforts by states to enhance their security can decrease the security of others. The severity of a security dilemma, and the prospects for cooperation under the dilemma, are greatly affected by military technology. In this article, I apply the security dilemma framework to a revolutionary new form of conflict: cyberwarfare. I argue that cooperation over cyberwarfare is made challenging due to the security dilemma, and that the unique characteristics of cyberwarfare make it difficult to break out of this dilemma. The reluctance and failure of states to achieve cooperation over cyberwarfare likely reflects, in part, the constraints of this “cybersecurity dilemma.” Some states have strong incentives, however, to promote limitations on offensive cyberwarfare. Thus, I propose ways in which cooperation may eventually be achieved despite these challenges