The dissertation is divided into two parts. Part I will concentrate on the theoretical debate and will put forward hypotheses on the functioning of security institutions. This section updates the classical literature on collective security with contemporary contributions and original insights and it attempts to shed light on the limits of the current debate between paradigms on the question of international institutions. Chapter 2 will summarize the arguments of the main paradigms. The two main positions - the neorealist/pessimistic one and the liberal/optimistic one - will be analyzed and a new concept of the conditional utility of institutions in international relations will be proposed, attempting to overcome the "all or nothing" deadlock of the current debate. The other three chapters of Part I will concentrate on the central concept of the work. Chapter 3 will introduce the various definitions of collective security and their position within the wider contest of the theory of international relations. In particular, it will be argued that there are two different conceptions of collective security. A maximalist one defines the concept as a security system replacing all other mechanisms. A minimalist one sees collective security as an international regime which can operate alongside other mechanisms. Chapter 4 will analyze the limits and the shortcomings of the maximalist conception, which is both unrealistic and even counterproductive. Chapter 5 will look at the positive effects of collective security either as an instrument for dealing with specific contingencies or as a general framework for facilitating cooperation and improving international standards of behaviour. Part II applies the findings of the first section to the historical record and to three case studies: the Abyssinian crisis, the Korean War and the Gulf Conflict The cases were selected because they are the only uncontroversial instances in which collective security has been applied and because they are conveniently placed in three different international systems: the multipolar inter-war period, the bipolar Cold War and the post-Cold War period. Finally, in the conclusion, crucial issues for practitioners will be highlighted with special reference to the prospects for a more stable system in the future.