The argument of lex mercatoria - because of its important implications mainly in the international and commercial field of great interest to the jurist of civil law - is also fundamental to the historian of law. In fact, it can be considered both as a witness of new commercial legal institutions risen from the practice of affairs and defined by an international juridical science, and as a moment of crisis of the consolidated system since the first codes of the juridical sources. The authors of the articles collected in the present volume are historians of law of different cultural background and provenience. The publication at issue was conceived as an almost obligatory intervention in a debate which rather scantily considers epistemology as well as disciplinary boundaries.Each single study highlights a different aspect of the lex mercatoria and its relationship to the ius commune, studying both under different perspectives. The authors explore well-founded historical evidence across a broad chronological period from the Middle Ages until the nineteenth century, acrossing institutional settings differing both politically and operationally.The historical problem of the lex mercatoria is mainly dealt with from the point of view of the sources. The volume collects general studies in relation to the problem of the existence of the lex mercatoria and more specific items - many of them dedicated to the maritime law. Thus different keys of interpretation are given concerning the development of the European commercial law.