The general international law regarding foreign merchant ships in internal waters has never been codified. Despite the codification efforts made by the League of Nations and the Geneva Convention on the Territorial Sea and the Contiguous Zone of 1958, the question of the breadth of the territorial sea was finally solved during the Third United Nations Conference on the Law of the Sea. In addition, the right of innocent passage of foreign ships has been regulated in the 1982 Convention in greater detail than ever before, whilst the new regime on the prevention of marine pollution in the Convention has also considerable impact on this right. In practice, potential conflicts between coastal States and foreign merchant ships in internal waters and the territorial sea may well arise. This comprehensive study analyses these issues and cautiously strives for reasonable and generally acceptable solutions.