The chapter analyses International Court of Justice(ICJ) environmental law cases to demonstrate that judges have less competence in such matters. Such issues are technical and complex, and changing scientific knowledge and uncertainties make adjudication challenging, leading to vague norms in international environmental law. However, procedural and substantive solutions are available to adjudicators faced with scientific uncertainties. Among other solutions is use of technical experts who can deliver expert evidence under art 50 ICJ Statute. Substantive law has progressed in response to complex ecological problems through the precautionary principle. By applying the principle, policy makers undertook to reconcile the divergence between law and science. It aims to provide guidance on development and application of international law where there is scientific uncertainty; multilateral environmental agreements provide for this. Nonetheless, possible interpretations of the principle have rendered it vague. It is postulated that, like other international environmental law principles, it is vague due to unscientific uncertainty.