The law merchant has been described as “a venerable old lady who has twice disappeared from the face of the earth and twice been resuscitated” (Goldman, 1983: 3). Berthold Goldman here refers to the disappearance of the Roman jus gentium, occasioned by the breakup of the Roman world and both the disintegration of international economic relations in the Middle Ages and its reappearance in eleventh-century Europe with the rebirth of international commerce. The second disappearance is said to have occurred in the seventeenth century when nation-states localized and nationalized commercial law, resulting in a period of hibernation that lasted until the reappearance of a modern lex mercatoria in the twentieth century. Legal theorists generally agree that the law merchant has progressed through three phases in its evolution, although few would agree that it had ever “disappeared from the face of the earth.” However, the characterization of its development as a continuous movement or progression from the first phase of medieval internationalism, through the second of localization and nationalization, to the third of modern internationalism and transnationalism misses sharp discontinuities and changes flowing from altered historical, material, ideological, and institutional conditions. The law merchant is implicated in significant changes in society and political economy over its three phases. It has changed in relation to changing social, political, and economic conditions. But it has also contributed to their transformation. This chapter analyzes the first phase in the development of the law merchant.