Today's security environment is characterized by change and ambiguity. The future will include a variety of challenges, including the risk of catastrophic attacks on the homeland, and the possibility of disruptive technological breakthroughs by our adversaries. The number and character of potential U.S. adversaries is growing and changing, as states and non-state actors acquire advanced technology and even weapons of mass destruction. We can foresee the near-term threats posed by ballistic and cruise missiles; chemical, biological, radiological, and nuclear weapons; advanced double-digit surface-to-air missiles; and sophisticated combat aircraft. We should also anticipate computer network attacks and attacks on other critical infrastructure, including space networks. Not only must we be prepared to confront these known threats, but we also must be ready for unexpected, disruptive breakthroughs in technology that may undercut traditional U.S. advantages. Maintaining a strong defense able to overcome and defeat these threats remains an imperative for our nation. Currently, the Air Force can command the global commons of air and space, and significantly influence the global commons of sea and cyberspace; however, we cannot maintain this advantage using yesterday's technology in the systems and air and space vehicles of our current force structure. Recapitalizing our aging systems is our number one challenge.