This book examines the law relating to the possession, threat or use of nuclear weapons. By addressing in logical sequence the law regarding sovereignty, the threat or use of force, the conduct of nuclear hostilities, neutrality, weapons law and war crimes, the book illustrates the topics that an effective national command, control and communications system for nuclear weapons must address. Guidance is given on intractable issues, such as the responsibilities of remote submarine commanders. The continuing relevance of the ICJ’s Nuclear Advisory Opinion is assessed, and the prospects for the Treaty on the Prohibition of Nuclear Weapons are discussed. The book has been written in an accessible style so that it will be equally useful to lawyers and practitioners, including relevant commanders, politicians, policy staffs and academics. The objective is to state the law accurately and to explain its implications and provide practical guidance in this most sensitive area. This book is also available as open access.