The post 9/11 innovations in the Security Council's counter-terrorist practice point in the direction of a metamorphosis in the functions of the Security Council, from a body addressing security threats to a body developing a criminal and security policy with both legislative and quasi-judicial functions. Focusing on two separate but related resolutions adopted by the UN Security Council under Chapter VII of the UN Charter, and their resulting counter-terrorist regimes, the first part of the thesis argues that the post 9/11 counter-terrorism measures adopted by the Security Council broke new grounds by stretching the boundaries of space and time which should limit the Security Council's action under Chapter VII of the UN Charter, and its specific, preliminary nature. The thesis then turns to investigate the Security Council practice in its post 9/11 counter-terrorist initiatives, through the work of its two Committees: the 1373 Counter-Terrorism Committee and the 1267 Sanctions Committee. Through an in depth analysis into the day-to-day work of the Committees, the evolution in their working methods and their interaction with UN Member States, the thesis provides a detailed assessment of how the Security Council has affected international human rights standards in its counter-terrorist action. It also provides an investigation into the question of state compliance with the counter-terrorism measures adopted by the Security Council. The thesis concludes by contextualizing the implications of this post 9/11 counter-terrorist action in traditional doctrines of international institutional law and their normative underpinnings. With the Security Council increasingly infringing on human rights standards, and mechanisms of direct accountability still lacking at the international level, new mechanisms of accountability are developing through state opposition or non-compliance, at times driven by individuals themselves through the judicial mechanisms of the state. As such the Security Council's counter-terrorist action and the reaction to it sheds new light on the balance of powers between the Security Council and the broader UN membership, but also on different notions of constitutionalism, in which the individual and international human rights standards play a more prominent role.