This book provides a comprehensive and detailed analysis of the nature, content and scope of the rules regulating the use of force in international law as they are contained in the United Nations Charter, customary international law and international jurisprudence.The book's scope is broad and covers the prohibition on the threat or use of force; the use of force in self-defence; the use of force as part of the United Nations collective security system; the use of force by regional organisations; the use of force in peacekeeping operations; the use of force for humanitarian purposes; the use of force by invitation; armed reprisals; the use of force by and against non-State actors; and the use of force in cyberspace. The book takes an insightful look at the rules regulating the use of force as they are called upon to apply to changing and challenging circumstances such as the emergence of non-State actors, security risks, new technologies and moral considerations. Its arguments balance the interests of stability and change in order to enhance international law's regulatory potential regarding the use of force.This book is an important resource for students and scholars of international law, the use of force and collective security and for practitioners involved in the interpretation and application of these legal frameworks.