This chapter develops a sociological approach to legal institutions by looking at the genesis of the Court of First Instance (CFI). This European Court has remained largely under the radar of EU law studies. Research often consider the CJEU as a homogeneous and monolithic body and provides a functionalist explanation for the creation of the CFI: because of the case overload, a new court was needed to check the facts of complex cases. But this explanation is not sufficient to understand how a project, discussed since 1974, was added to the agenda of the intergovernmental conference only at the last-minute in 1985. Additionally, the first project, a Civil service tribunal was progressively abandoned in favour of a court with extensive competences, notably in competition law. Through the study of institutional archives with a sociological lense, the author shows that rather than a top-down decision, political mobilization was essential for the creation of the CFI. The chapter shows that the CFI creation was promoted by a coalition of transnational actors: EU officials and lawyers acting as legal entrepreneurs. Hence, the chapter highlights the need to reintegrate the CJEU and its members in broader networks of actors and institutional chains of interdependence.