The "Common Law" (ius commune) has definitively lost its practical significance with the enactment of the Civil Code. However, its concept is continuously mentioned in legal history as a core component of the theory of legal sources and in the context of the reception of Roman-canonical law. Its understanding remains of lasting importance for European private law history, as the general identification of Common Law with Roman, Saxon, or other sources of law is problematic, and the assertion that a specific law was considered Common Law at a certain time in a certain place only makes sense if it is clear what it means to be valid as Common Law. The dissertation from Trier, initiated and supervised by Peter Krause, contributes to the history of concepts without exhausting the topic. The first chapter presents controversies.