The chapter considers the nature of authorities in the law, and, specifically, the often undervalued yet important role played by informal auctoritas, by contrast with formally binding enactments. It explores the terminological difficulties within modern discourse which make discussion and understanding of such informal authorities difficult. Exemplarily, the status of Roman sources in the ius commune is considered in detail, establishing the informal criteria which determine their authoritative quality – belief in the extraordinary quality of the texts and the jurists’ mutual expectations of applying them. The analysis then proceeds to modern German practice and the enormously significant role played by learned Commentaries in the work of practising lawyers and courts – a phenomenon difficult to explain without recourse to a concept of informal authority. Finally, this understanding is demonstrated in the context of transnational private law by reference to the modern phenomenon of non-legislative codifications, such as the American Restatements, the Principles of European Contract Law and the UNIDROIT Principles of International Commercial Contracts; all those codifications have gained a substantial informal authority as reference texts for international legal discourse.