This chapter examines the use of lex mercatoria as a substitute legal system for transnational private conflicts. The lex mercatoria is a set of ad hoc, international rules that originate from non-state entities and govern business transactions outside of the conventional conflict of law system. In international arbitration, for example, arbitrators frequently rely on it in the lack of clear national legal frameworks. These transnational rules continue to offer adaptable solutions in international trade, addressing legal gaps and enhancing both domestic laws and international treaties, despite criticism regarding their regulatory scattering. The chapter explores the development of the lex mercatoria, its origins, and its empirical application.