This chapter examines the constraints and obligations that customary law of the sea and conventions sponsored by the United Nations (UN) have placed on nations and their navies. It assesses the potential for such issues as national sovereignty, maritime boundaries and economic uses of the seas to cause disputes that may lead to armed conflict today, despite or because of UN conventions. Two case studies will be used to present the potential for conflict between a sovereign state’s demands for security and other sea users’ claims for freedom of navigation. The Corfu Channel Incident of 1946, between Albania and Britain, will be used to show how different interpretations of customary law can lead to armed conflict and loss of life.1 The prolonged dispute between Greece and Turkey over jurisdiction in the Aegean Sea will be used to demonstrate both the imprecision of modern international law and the continuing inability of the International Court of Justice (ICJ) to arbitrate or impose a settlement without both parties’ consent. The Royal Navy has had a long tradition of being interested in and shaping International Law, and the nature of this law as it develops will have a bearing on future Royal Navy operations. Inevitably, conflicts that arise over jurisdictional disputes still have the potential to draw in HM Ships.