This book brings the legal rules governing the use of weapons in armed conflict together into a single volume and interprets and applies those principles and rules to particular weapons technologies. It is the essential reference book for anyone dealing or concerned with the international law applying to weaponry. After relating the historical evolution of weapons law, identifying its sources and discussing the important customary principles that are the foundation of the subject, the book explains to the reader in a logical sequence of chapters how treaty and customary rules apply to particular categories of weapon or to relevant technologies, both traditional and novel. Having explained to the reader how the existing law applies across the full range of weapons technologies, the book discusses how this dynamic field of international law may be expected to develop in the years ahead. This new edition tackles challenging weapons law issues such as the new treaty law on expanding bullets and on the arms trade, novel technologies in the fields of chemistry and biology, the topical controversies associated with autonomous and automated weapon systems, and how law applies to weapons in outer space and to cyber weapons. The law applicable in non-international armed conflicts is summarized; compliance and weapon reviews are carefully explained; and recent international and national military manuals, and other developments in the wider literature, are thoroughly reflected throughout the text.