This Festschrift is published on the occasion of Gerhard Hafner’s 65th birthday and his retirement as a professor at the University of Vienna. It assembles a great number of renowned friends and colleagues in international law honouring Gerhard Hafner’s outstanding career as scholar, diplomat, legal adviser and arbitrator. The diversity of areas selected for this Festschrift reflects the generalist approach of Gerhard Hafner towards international law. Among the topics on which his contribution was particularly influential are the fragmentation of international law, the law of State immunity and international criminal law, which feature prominently in the Festschrift. Other areas covered are the theory of international law (including sources), basic principles of international law, codification of international law, subjects of international law, international dispute settlement, the law of the sea and international environmental law, human rights and humanitarian law and the law of the European Union.