There are currently no rules in international commercial arbitration law and practice assuring the coordination between (partial or final) arbitral awards and/or national court judgments rendered in identical or related cases. This lack of coordination is unsatisfactory, particularly in light of the ever-growing tendency of parties to submit their commercial disputes to international arbitration and the increasing complexity of international arbitration. Today, international commercial transactions and the disputes to which they give rise regularly involve multiple parties, contracts and issues. As a consequence, these disputes (or certain aspects of these disputes) are increasingly tried in multiple fora. In such circumstances, difficult issues regarding the res judicata effects of prior judgments or awards are likely to arise before international commercial arbitral tribunals. The central hypothesis underlying this research is that transnational principles of res judicata should be elaborated for international commercial arbitral tribunals. This solution is justified for several reasons. First, it is justified given the differences among domestic laws regarding res judicata and the difficulties surrounding the formulation of appropriate conflict-of-laws rules. Second, it avoids inappropriate analogies between international arbitration proceedings and litigation. Finally, the solution provides guidance and ensures a certain degree of fairness, certainty and predictability, which is expected by arbitration users. This PhD thesis seeks to achieve its aims in two stages: Part One examines the doctrine of res judicata in litigation, analysing the doctrine as applied in different domestic laws, as well as in private and public international law. Part Two will determine whether and to what extent the res judicata doctrine may be applied by international commercial arbitral tribunals. It will demonstrate that transnational principles of res judicata should be elaborated and will seek to formulate such principles.