This volume has three parts: the first focuses on cyberspace itself; the second on some of the major forms of malevolence or threats that have become one of its defining characteristics; and the third on possible responses to these threats. One of the most significant features of cyberspace, however, is that it is becoming a risky place for the entire spectrum of users: nation-states, nongovernmental and transnational organizations, commercial enterprises, and individuals. Yet it is a space of opportunities—for benevolent, neutral, and malevolent actors. Moreover, the authors identify and assess the challenges and threats to security that can arise in cyberspace because of its unique nature. In the final section, the authors discuss a variety of responses, with some suggesting that the most favored options being pursued by the United States are poorly conceived and ill-suited to the tasks at hand.