Focused on the experts groups of the European Commission, this article aims to understand not only who the European experts are, but also on what grounds. It adopts a sociological perspective, paying attention to the social properties of those actors, in order to better grasp which type of actors can gain access to the European policy-making process by this way, but also how the experts' authority can get the upper hand in the European arena, and under what conditions it may be exerted. The article begins with an overview of the space of European expertise: highlighting the political uses of the groups to explain the privileged recruitment of experts in certain categories of practitioners ('academics', members of interest groups, national civil servants and so on). These political uses of expertise also contribute to promoting particular resources and practices, leading to analysis of the properties shared by the experts, in spite of their apparent heterogeneity. A number of these properties are moreover acquired in the European space, inviting to take a closer look at the way these expertise functions are fulfilled in professional trajectories associated with EU institutions.