The explanatory capacity of ideas has been contested on two grounds. First, ideas have been dismissed as epiphenomenal. Second, ideational explanations have been criticized for the limited importance that they ascribe to agency. This article examines the involvement of the European Commission in previously uncharted territory, namely the regulation of professional sport in Europe. It demonstrates that, in conditions of ambiguity and uncertainty created by the need to implement broad Treaty-based principles in new areas of socio-economic activity, ideas, first, act as road maps that direct the executive activity of the European Commission, legitimize it, and set limits to it by identifying the relevant deeply embedded conceptions of the nature of a given activity and by linking them to a wider, historically defined normative order. Second, ideas are also powerful political weapons used by political actors in their quest to advance their interests.