The demilitarisation and neutralisation of the Åland Islands is a confirmation of, and an exception to, the collective security system in present-day international affairs. Its core idea is that there is no need for military presence in the territory of the islands and that they are to be kept out of military activities. Through parliamentary and archival materials, international treaties and academic works, the authors examine the legal rules and institutional structures of the demilitarisation regime. In this process they reassess core concepts of international law and international affairs, such as sovereignty and security, and introduce a theoretical view on the empirical case study of the Åland Islands. The book covers legal, political and policy discursive aspects of demilitarisation, international co-operation, defence and security matters around the Baltic Sea with a broader European and global relevance.