Even before the terrorist attacks of September 2001, concerns had been rising among security experts about the vulnerabilities to attack of computer systems and associated infrastructure. Yet, despite increasing attention from federal and state governments and international organisations, the defence against attacks on these systems has appeared to be generally fragmented and varying widely in effectiveness. Concerns have grown that what is needed is a national cybersecurity framework a co-ordinated, coherent set of public- and private-sector efforts required to ensure an acceptable level of cybersecurity for the nation. As commonly used, cybersecurity refers to three things: measures to protect information technology; the information it contains, processes, and transmits, and associated physical and virtual elements (which together comprise cyberspace); the degree of protection resulting from application of those measures; and the associated field of professional endeavour. Virtually any element of cyberspace can be at risk, and the degree of interconnection of those elements can make it difficult to determine the extent of the cybersecurity framework that is needed. Identifying the major weaknesses in U.S. cybersecurity is an area of some controversy. However, some components appear to be sources of potentially significant risk because either major vulnerabilities have been identified or substantial impacts could result from a successful attack in particular, components that play critical roles in elements of critical infrastructure, widely used commercial software, organisational governance, and the level of public knowledge and perception about cybersecurity. This book addresses each of those questions in turn.