This book concerns a topic of increasing importance in international dispute settlement: the principles according to which an international tribunal with a limited jurisdiction may apply 'law' other than the law specific to its jurisdiction when resolving a dispute. These questions are sometimes regulated in the instruments establishing particular tribunals, but the interpretation of these provisions has proved to be controversial, while many instruments establishing international courts and tribunals fail entirely to provide guidance on these issues. This book has three main objectives: first, to undertake a comprehensive theoretical and comparative study of the treatment of the issue of applicable law in the most important instruments establishing international courts and tribunals; second, to arrive at default rules that should apply to tribunals without an express rule on applicable law; and third, to explore the implications of such rules for the international legal order. The conceptual context of the research question at issue is the increasing fragmentation of the international legal order, which is a result of the proliferation of both international treaties and international dispute settlement activity over the past decade. In this overall context, there have been studies on different aspects of this issue, including the scope of jurisdiction of international tribunals, conflicts between the decisions of international tribunals and - this being an old question - the hierarchy of norms in general international law. However, this book is the first to give a comprehensive and comparative treatment to the topic of the 'applicable law' of specific courts and tribunals.