Nowadays traders are very frequently conducting their businesses in accordance with principles and usages forged in the practice of commerce. This has given rise to an ongoing discussion on the existence of an autonomous third legal order called transnational commercial law or the lex mercatoria. This article looks at the role of good faith in that legal system. As a consequence of the evolution of the law of contracts, the rise of transnational law and of the influence of its prevalent actors - multinational corporations - a cooperative view of contracts has been developed in international trade. This article argues that the rationale of cooperation, as the underlying current of transnational commercial contracts, has prompted a new way of interpreting the principle of good faith: it is understood as cooperation between the parties to a contract. This interpretation of good faith requires the party to take various steps to fulfil the legitimate expectations of the other party. Rather than being imposed by a central authority, such a predominantly voluntary cooperation is assumed by the parties for the common good of everyone involved in the contractual relationship. This notion fits the experience of global trade today to the point that - it will be submitted - good faith is the fulcrum of cooperation in cross-border trade. This proposition will be supported through the analysis of: philosophical doctrines; principles embracing transnational law and international arbitral awards. Furthermore, the development of good faith in some municipal legal systems will be considered; as well as the latest developments of good faith in EU law.