This dissertation theorises the concept of coherence in international law, with an emphasis on international investment arbitration. Coherence is here characterised as a meta-principle whose presence is both pervasive in the international legal argument and necessary for the justification of judicial pronouncements. However, the principle of coherence is not just a restatement of the idea of systemic integration in the sense of Article 31(3)(c) of the VCLT. For, even though coherence understood in a systemic sense is a necessary condition for justified judicial outcomes, it is still not a sufficient one. The principle of coherence also encompasses an axiological dimension, pointing towards morality and just outcomes, that supplements its systemic dimension. Therefore, calls for coherence in investment arbitration do not translate into a determinate result or end-state of affairs, but into an ideal state of affairs where positivist values (authority, certainty/predictability, and efficiency) perfectly co-exist with non-positivist ones (moral correctness or substantive justice). Because coherence describes something that will always be beyond our grasp, its value for judicial reasoning may also lie in it being used as an epistemic tool during a tribunal’s process of deliberation. Following the principle of coherence as epistemic tool will not guarantee that the outcomes reached will always be the correct ones; nor will it always guarantee predictability or certainty as to how a case might turn out. What coherence can achieve, however, is that an outcome that is sensitive to its dual nature will appear epistemically objective and thus legitimate.