This book examines the extent to which it is possible to identify a coherent legal method (doctrine of the sources of law and their interpretation) that may be applied when analysing EU law and the law of EU Member States within the scope of application of EU law. It looks at what characterises the sources of law and the interpretation methods that are actually used by European legal actors, especially judges and researchers, when analysing what is valid law. It examines the changes in the relative importance of various sources of law that occur in connection with the integration of EU law into national law.