This chapter discusses the nature of cyber threats against government and private computer systems, describing some steps the government has taken and the challenges involved in protecting those systems. The chapter argues that a national security approach for cyber security policy is the most promising option for preventing these cyber threats while operating within the domestic legal framework. After a review of the President's constitutional authorities to protect the nation from traditional threats, the chapter concludes that the President has some power to monitor Internet communications in transit within the United States when the communications threaten the welfare of the nation. The chapter recommends that this authority be augmented by Congressional action through legislation. The President's powers in cyber security, even given Congressional support, however, are still restrained by the protections the Fourth Amendment provides for traditional forms of communication and individual privacy. Although there is limited Fourth Amendment precedent in the area of cyber security, the well-established exceptions to the Fourth Amendment requirements, based on consent, special governmental needs and the reasonableness of the search or seizure, provide a legal basis for executive branch action to protect critical infrastructures and their computer systems. As the Courts have long held, these exceptions allow the government to conduct searches or seizures without being bound by all of the requirements of the Fourth Amendment. If the government develops its cyber security policy in line with these exceptions, this chapter argues the government can both protect critical computer systems and operate within Fourth Amendment doctrine that recognizes the legitimacy of privacy in electronic communications.