In the course of a single investor-state dispute, an arbitrator may make numerous decisions, from interpreting the treaty or national laws to taking into account case law, customs and policies. In practice, this process raises important issues regarding the consistency of decisions and the predictability and legitimacy of the decision-making process in general. Investment arbitration tribunals have developed a specialised process of legal decision making adapted to the interpretational needs that arise in the context of an investor-state dispute and to the transnational characteristics of the investment arbitration framework. This is the first book to offer an in-depth analysis of the transnational characteristics of investment arbitration and to analyse the interpretive arguments of investment tribunals and the way they use treaties, precedent, policies, general principles of law and customary law in their decision-making process. Drawing on publicly available arbitral case law supplemented with personal interviews with investment arbitrators, the author touches on such concepts and practices as the following: - an overview of various decision-making genres of arbitral tribunals: attitudinal, economic, strategic and legal; - the legal argumentation triptych of language–rhetoric–dialogue; - the specific language arbitrators have developed when interpreting the law; - how arbitrators use the concepts 'standards', 'rules', 'principles' and 'rights'; - the importance of the legal reasoning of arbitral awards and the role of rhetoric therein; - concepts of 'acceptability', 'audience' and 'legitimacy'; - limitations of the public international law interpretive methodology enshrined in the Vienna Convention; - interpretation of precedents, customary law, general principles of law and policies; - the way national and international legal orders interact in the context of interpretation; and - how decision-making is connected to the issues of predictability, consistency and the rule of law. The core of the book proposes a novel, full- edged dialogical network theory for analysing the interpretation process. As an exemplary demonstration of developing theory to keep up with practice, this unique book provides a deeply engaged means for enhancing the practice of international arbitration. Its introduction of a new field of interdisciplinary analysis employing legal argumentation theories is sure to provide inestimable guidance for institutions and policymakers, especially in light of recent proposals for the creation of a permanent investment arbitration court. Given that unveiling the legal decision-making process is critical for the well-being of the whole dispute resolution procedure, and that being aware of how arbitrators interpret the law can constitute a roadmap for counsel's arguments and approaches when dealing with cross-border disputes, the topic of this book is relevant for both academics and practitioners, and its signifcance can only grow as recourse to investor-state arbitration continues to expand.