The notion that Civil Law countries are monist, while Common Law countries are dualist is challenged in this study. It looks at the application of treaties and of European Community Law in States belonging to either system. In each State, the solutions depend on particular factors. However, in all cases, national judges often hesitate to apply rules of international law, even within the framework of the European Union. This being said, domestic solutions cannot simply be explained on the basis of the legal system in which they are adopted. Indeed, when it comes to the application of treaties within their legal order, some Civil Law countries are dualist.