This book brings together updated papers presented at a conference to commemorate the 40th anniversary of the conclusion of the Vienna Convention on the Law of Treaties. The collection consists of nine articles by scholars with a strong interest in the areas of state responsibility and treaty law. As the interplay between state responsibility and treaty law becomes more and more important, a book dealing with this topic cannot fail to be of interest. Contents include: Does the Difference Make a Difference? A Comparison between the Mechanisms of the Law of Treaties and of State Responsibility as Means to Derogate from Treaty Obligations in Cases of Subsequent Changes of Circumstances * 'Article 98' Agreements, the Law of Treaties, and the Law of State Responsibility * State Responsibility and Private Military Companies * The Duty Not to Recognize as Lawful a Situation Created by the Illegal Use of Force: From Kosovo to Abkhazia and South Ossetia * Jus Cogens: A Puzzle * Applying the Law of State Responsibility to Military Counter-Terrorist Actions * Genuine Test of Genuine Prosecutions? A Human Rights Model for Complementarity * Treaty Interpretation and Tottering Treaty Regimes: The Case of International Investment Law * International Legal Responsibility of the European Union According to the Draft Articles of the International Law Commission