The increasing complexity of litigation and increasingly frequent use of scientific expertise raises questions about the role of non judicial elements in jurisprudence. In the environmental field, the frequent recourse to expertise leads to a paradox: while the increasing importance of scientific considerations almost systematically calls for scientific expertise, the scientific uncertainty, coupled with the political stakes makes it difficult for a judge to use this expertise. Due to the impossibility for a judge to isolate himself from scientific controversies, the expert has become a prime stakeholder in environmental and health issues and subject to criticism. The relationship between the expert and the magistrate thus becomes highly important. International jurisdiction is more and more often called upon to validate the scientific basis of the judgements of lower courts. This is also the case of the World Trade Organisation. As dispute settlement panels assess the value of a scientific expertise or the accuracy of a risk assessment, they become judges of scientific expertise. What are the modalities of the use of scientific experts? What is the importance of the scientific expertise in the making of a ruling? These questions will allow us to contribute to the discussion about the increasing use of scientific expertise in judicial proceedings at the World Trade Organisation.