Most armed conflicts today are non-international. These are conflicts in which one or more organized non-State armed groups (NSAG) are fighting either between themselves or against State armed forces. The International Law of Non-International Armed Conflict offers a clear and practical analysis of the application of international humanitarian law (IHL) and international human rights law (IHRL) to limit the effects of non-international armed conflicts (NIACs). This book examines the threshold for the existence of a NIAC and the application of IHL, and explores how treaty and customary rules balance military necessity with humanitarian concerns to regulate the conduct of hostilities and protect persons not participating in the fighting. It also addresses contemporary challenges, such as counter-terrorism operations, environmental protection, and the use of new technologies. Furthermore, the book examines the application of IHRL, particularly in light of the millions living under NSAG control and de facto governance. It also explores mechanisms for ensuring the parties' compliance with international law and accountability for violations, including developments in international criminal law, as well as State and corporate responsibility. Rich in legal analysis and grounded in practice, this volume is an essential resource for scholars, legal practitioners, and advanced students seeking a comprehensive understanding of the legal dimensions of modern armed conflict.