The COVID19 pandemic has shed light on the central role of scientific expertise in time of health crisis, in a context where trust in expertise is a pressing issue. The need for timely public action is often in tension with the production of robust scientific knowledge. Consequently, the call for expertise appears problematic on political (legitimacy) and epistemic (credibility) levels. Based on real-time data collection analysed with the tools of Science and Technology Studies, the EXPERCRISE project examines the following questions : What are the elements that ensure the credibility and legitimacy of scientific expertise for decision-making ? The project will provide a case study of the French situation. This case study will be included in an international comparative initiative coordinated by Steven Hilgartner (Cornell University) and Sheila Jasanoff (Harvard University) and involving partners in 10 countries. The objective of the comparison is to identify national specificities in the definition and use of credible and legitimate expertise, and to connect these specificities to historical, regulatory or organisational characteristics. The results of the international comparison, as those of EXPERCRISE, will be available as a series of country reports published online and for immediate use by decision-makers. They will be published as a collective volume available in open access.