The ‘War on Terror’ poses great challenges to democracies. The need for appropriate counter-terrorism measures prompts greater discretion to the executive in enacting adequate measures. This thesis demonstrates that since September 11th 2001, the US and the UK have both modified and enacted further national security measures and anti-terrorism legislation accordingly. The thesis critically explores and examines the legality and constitutional legitimacy of the means that the US and UK have used, giving equal normative weight to national security and civil liberty imperatives. The focus is not merely between how this legislation strikes a balance between national security and civil liberties; it is also on the ways in which judicial intervention serves as a check to both executive and administrative measures. Thus, it is always necessary to conduct a balancing act between the rights of citizens to live in peace and security of persons and property, against the rights of individuals who may seek to threaten this. The core argument is that the history of both countries indicate that adhering to a human rights standard as far as possible, even in times of war, has both military and political benefits. Nevertheless, it is not claimed that all rights must be protected at all times. Instead, it seeks to establish a legal standard that recognises the need to protect human rights whilst also protecting security The thesis then turns to an examination of the judicial determinations in times of conflict between national security and civil liberties. The conclusion is that protecting national security interests is at the heart of all nations, however both the executive and the judiciary as upholders of the rule of law, should ensure the action taken is both adequate and necessary in the sense of proportionate (no more but also no less than what is called for). Thus, it calls for greater judicial oversight and accountability of executive action by using, as far as possible, the ordinary rules of criminal justice to deal with suspected terrorists. Furthermore, it is argued that to follow such a long term approach would provide an adequate platform for other countries in the struggle against terrorism. The thesis also identifies many lessons that can be learnt from the approach of the two countries.