In the globalised world of today, states face increasingly complex and transnational threats, which include, among other things, terrorism, international migration, weapons proliferation, poverty and human rights violations. This book maintains that the most effective responses to such threats are multilateral ones -- an argument that is supported by an exploration of the various multilateral organisations and instruments of security. The theoretical foundations for this book are set out in The Five Dimensions of Global Security: Proposal for a Multi-sum Security Principle (2007), and Symbiotic Realism: A Theory of International Relations in an Instant and an Interdependent World (2007). The former proposes five dimensions of global security (human, environmental, national, transnational and transcultural) that depend on good governance and justice. The latter proposes a new theory of international relations that addresses the instant and interdependent nature of our globalised world and enlarges the number of actors beyond the traditional state and non-state actors. Our instant and interdependent world makes multilateral responses better suited to current global security threats, and means that only multilateral responses can provide the authority, legitimacy, resources and burden-sharing that are necessary to tackle these threats -- because they advance a more just and sustainable world order.