How much do European Union (EU) interventions influence national public policies? Does the EU intervene in a “subsidiary” way, as asserted in the Community Treaties, or is it omnipresent and omnipotent as public opinion sometimes claims? For example, is 80% of national legislation of Community origin or, to the contrary, is it a much smaller share? At a time when EU citizens have just elected their representatives in the European Parliament for the next five years, it seems more necessary than ever to try to address these questions which are essential from a policy vantage point, but which have not yet generally given rise to very precise responses.