The general theme of this volume of the Yearbook of International Humanitarian Lawis armed groups and the challenges arising from the participation of such groups incontemporary armed conflicts. It is elaborated upon in several chapters, addressingthe organisation criterion, respect for and compliance with international humanitarianlaw and international human rights law, targeted sanctions and accountability issues,among other things. Besides these chapters that can be connected to the general theme,the book also contains a chapter dedicated to the ‘knock on the roof ’ practice, a Year inReview, describing the most important events and legal developments that took place in2016, as well as the final report from the ILA Study Group ‘The Conduct of HostilitiesUnder International Humanitarian Law - Challenges of 21st Century Warfare’.The Yearbook of International Humanitarian Law is the world’s only annual publicationdevoted to the study of the laws governing armed conflict. It provides a truly internationalforum for high-quality, peer-reviewed academic articles focusing on this crucialbranch of international law. Distinguished by contemporary relevance, the Yearbookof International Humanitarian Law bridges the gap between theory and practice andserves as a useful reference tool for scholars, practitioners, military personnel, civilservants, diplomats, human rights workers and students.