Over the last 30 years, the Vienna Convention on the Law of Treaties1 has emerged as one of the most influential instruments of modern international law. The Convention, which was adopted at the UN Conference on the Law of Treaties on 23 May 1969, entered into force on 27 January 1980 and has meanwhile been ratified by more than 80 States.2 Yet, as it does not operate retroactively,3 the scope of application is growing only slowly and its practical importance stems, rather, from the fact that the Convention is widely considered a restatement of customary international law. As early as 1971 the International Court of Justice referred to the articles governing termination for breach of treaty as a codification of the existing law on the subject.4 Since then both international tribunals and national courts have more and more habitually relied on the material provisions of the Convention to ascertain traditional rules of the law of treaties.5