Along with the rest of the U.S. government, the Department of Defense (DoD) depends on cyberspace to function. DoD operates over 15,000 networks and seven million computing devices across hundreds of installations in dozens of countries around the globe. DoD uses cyberspace to enable its military, intelligence, and business operations, including the movement of personnel and material and the command and control of the full spectrum of military operations. The Department and the nation have vulnerabilities in cyberspace. Our reliance on cyberspace stands in stark contrast to the inadequacy of our cybersecurity -- the security of the technologies that we use each day. Moreover, the continuing growth of networked systems, devices, and platforms means that cyberspace is embedded into an increasing number of capabilities upon which DoD relies to complete its mission. Today, many foreign nations are working to exploit DoD unclassified and classified networks, and some foreign intelligence organizations have already acquired the capacity to disrupt elements of DoD's information infrastructure. Moreover, non-state actors increasingly threaten to penetrate and disrupt DoD networks and systems. DoD, working with its interagency and international partners, seeks to mitigate the risks posed to U.S. and allied cyberspace capabilities, while protecting and respecting the principles of privacy and civil liberties, free expression, and innovation that have made cyberspace an integral part of U.S. prosperity and security. How the Department leverages the opportunities of cyberspace, while managing inherent uncertainties and reducing vulnerabilities, will significantly impact U.S. defensive readiness and national security for years to come.