Through further technological development and increased globalization, conducting busines abroad has become easier, especially for Small and Medium Enterprises (SME). However, the legal issues associated with international commerce have not lessened in complexity, including the role of non-state rules. The book provides a comprehensive analysis of non-state rules in international commercial contracts. Non-state rules have legal authority in the national and international sphere, but the key question is how this legal authority can be understood and established. To answer this question this book examines first what non-state rules are and how their legal authority can be measured, it then analyses how non-state rules are applied in different scenarios, including as the applicable law, as a source of law, or to interpret either the law or the contract. Throughout this analysis three other important questions are also answered: when can non-state rules be applied? when are they applied? and how are they applied? The book concludes with a framework and classification that leads to a deeper understanding of the legal authority of non-state rules. Providing a transnational perspective on this important topic, this book will appeal to anyone researching international commercial law. It will also be a valuable resource for arbitrators and anyone working in international commercial litigation.