This Commentary offers detailed background and analysis of the Treaty on the Prohibition of Nuclear Weapons, which was adopted at the UN Headquarters in New York in July 2017. The Treaty comprehensively prohibits the use, development, export, and possession of nuclear weapons. The treaty is examined article by article, with discussed of how each provision was negotiated and what it implies for states that join the Treaty. As the Treaty provisions cut across various branches of international law, the Commentary goes beyond a discussion of disarmament to consider the law of armed conflict, human rights, and the law on inter-state use of force. It examines the relationship with other treaties addressing nuclear weapons, in particular the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). Background on the development and possession of nuclear weapons and theories of nuclear deterrence is also provided.