Over the past one hundred years, the conceptual and legal aspects of collective security have been the subject of much debate. Rapid developments within the United Nations, its precursor the League of Nations, and regional security institutions, as well as the interaction between them, mean this debate has not so far succeeded in capturing the essence and implications of collective security. These developments in State and institutional practice strike at the heart of the entire system of collective security, which consists of universal, regional, and sub-regional levels, and indicate how the relationship between these various levels should be construed. Although the idea of collective security has raised high political expectations, it has always been based on legal instruments. Consequently, legal principles determine how far the delegated powers of collective security institutions extend and how the competencies of the United Nations relate to those of regional organisations. This book demonstrates that this inter-level interaction could find its expression in cooperation as well as confrontation between various collective security institutions, and influence the scope of competence of relevant collective security organs. This process then reinforces the concept of the unity of the multi-faceted system of collective security within which no institution has the power to conclusively define or interpret its own competence or that of other institutions. The book's originality lies in its dynamic and decentralised approach that focuses on the interaction between the different levels of collective security and in its comprehensive coverage of all pertinent institutions, competences, and relevant practice.