In France, judicial expertise operates within a specific institutional framework at the same time as it covers a distinctive community of practitioners called upon for their technical or scientific knowledge to serve justice. Indeed, while experts in the US are selected by the litigants, the French model features judge-appointed experts. This model could offer better guarantee of independence and neutrality, to the point that recent developments in the US suggest the emergence of a new court-appointed expert. What does such an institutional model involve in terms of evidence production? To answer this question, this paper looks at two areas of expertise in France: economic experts and forensic pathologists. Through an ethnography of the co-production of legal evidence, it analyses the black box of the French practices of legal expertise and allows the way in which the institutional context influences the producing of legal evidence, beyond differences between a scalpel and a calculator, to be understood.