The legal conformation of science is not universal, as reveals a comparison with the legal system of the United States. Both the French and the American systems are facing the problem of demarcation in forging a legal construction of science. A properly legal epistemology is to be seen in legal rules on expertise. Indeed, determining which expert is allowed to testify in court leads to determining what will count as science in the eyes of the legal system. Selecting scientific experts induces a choice on what might be ranked ‘good science' and what must be termed ‘junk science'. Thus, a plurality of legal constructions of ‘good science' has to be described.