In the late 1950s and early 1960s, the idea that a system of international commercial law was gradually replacing domestic laws in the sphere of international transactions began to interest a group of European scholars. Using a bygone expression, they called this phenomenon lex mercatoria, or “law merchant.” The exploration of lex mercatoria also coincided with the emergence of a full-fledged school of thought – what could be described as the French school of international arbitration. As this chapter shows, this was a time of intense renewal, which carried great appeal and led to bold, cutting-edge research. At the same time, vacillating between renewal and anxiety, many scholars strongly disagreed with the existence of lex mercatoria and voiced their disagreement. The first section of this chapter sketches the intellectual history of lex mercatoria; the second investigates its relationship with a nascent school of thought in international arbitration; and the third looks at the quarrels over lex mercatoria that marked the movement from renewal to anxiety.