When it comes to building European public action, expertise is ubiquitous and polymorphic. This article intends to study the ways expertise is being used in the European Commission and the logics underlying its use. The massive use of expertise also has consequences for the practices and identities of actors with whom European institutions coproduce public policies. On these subjects, the current debates on how experts are selected who oppose the Commission, European parliament members and NGOs, offer a stimulating field of investigation.