Soft law in international trade, and to an extent in similar international relations, is subject to certain paradoxes. The lex mercatoria doctrine divides the academic community, not only as to the conceptual question but also the implications in terms of the relevant applicable law to the potential dispute. However, the case is different for respective individual non-state rules. Their existence is accepted and highly valued both in terms of their use in contracting and formulation of the contract’s content, as well as in court or arbitral tribunal decision-making. The impending fundamental discrepancy is the degree of conceptualism in the evaluation of each of the non-state rules and their subsequent transposition into a coherent doctrine that seeks to compete with the existing private international law concepts. Lex mercatoria, or the transnational law of international trade (as a more appropriate name would perhaps be for a concept detached from national laws), has emerged in strong interaction with international practice. The emergence was, and still is, both spontaneous and unwritten, as well as formulated in various legal rules. Such rules’ essential characteristics are their familiarity, applicability, and actual usage in international trade practice. However, in all forms, they originate outside the regular legislative process. It is precisely this deficit of proper law-making that, among other things, can be used against the very existence of the lex mercatoria doctrine. The question is no longer “whether” lex mercatoria exists. The rules, at least in their soft law form, do exist, are identifiable, and are commonly applied. They are observably used in the contracting practice and decision-making. The question is instead “when” and “under what conditions” is the lex mercatoria applied. This book presents two fundamental concepts of understanding the non-state rules, as they appear in the field of international trade –integrative and transnational.