The computer age has opened up new vulnerabilities and attack options for potential adversaries. Unprecedented dependence on information and communications systems, computers, and networks demonstrate two new realities: United States infrastructure is vulnerable to attack; and the tools to conduct these cyber assaults are widely available, broadly known, and easily used. The ability of the United States to maintain, deploy, and sustain military power depends on the stability of domestic infrastructures owned and operated by the private sector. Better than 90 percent of essential military communications are carried over the public switched network. In 1995, Presidential Decision Directive (PDD) 39 instructed a cabinet committee to review critical national infrastructure’s vulnerability to terrorism in order to make recommendations to the President. The President’s Commission on Critical Infrastructure Protection (PCCIP) was designed to report to the President on threats involving vulnerabilities to critical national infrastructures while providing policy alternatives and solutions. Interlinked computer networks regulate the flow of power, water, financial services, medical care, telecommunications networks, and transportation systems. Defense planning must include a focus on how the domestic critical infrastructure can be protected from electronic attack, manipulation, or exploitation. This threat may originate from a variety of sources, including nations, intelligence services, terrorist or organized criminal groups, economic competitors, hackers, vandals, and disgruntled employees. Threat sources often cannot be tracked and identified because there are no indications and warnings systems for cyber attacks; attacks require only access to a computer and a modem. Eventually a partnership between defense, law enforcement, other government functions, and the private sector will be essential and inevitable to protect against cyber attacks.