EU law and the national laws of the EM Member States are closely interwoven. The implementation of EU law usually requires a recourse to the legal rules and concepts of national law. Since EU regulations are directly applicable, the national judge applying the law is confronted with thoroughly different problems than when applying purely national law based on European directives. These problems have long been neglected, both practically and theoretically. Following the analysis of these particular issues, the author develops a methodical approach to deal with the application of regulation law in a multi-layered legal system. This concept is then applied to selected GDPR example cases, which in addition gives new insights into current legal issues in data protection law.