This article aims to show how experts in European law shaped the direction and content of European public policy in the area of criminal justice. Putting policy experts at the core of our analysis allows us to examine the meso level of policy-making processes directly, where we can study the actors that played a concrete role in providing political and technical solutions during the draft- ing and adoption of European decisions. Our research focuses specifically on magis- trates and senior civil servants who were actively involved in formulating and adopting the instruments of judicial co-operation (the Eurojust unit and the Euro- pean Arrest Warrant). Through a sociological analysis of individual and collective actions of these legal professionals, we can explain how this branch of the EU was institutionalized and achieved its current form.