This is a legal commentary on the most recent global disarmament law treaty. Born out of deep concern about the humanitarian consequences of any use of nuclear weapons and a frustration at the refusal of nuclear-armed powers to countenance nuclear disarmament, the 2017 Treaty on the Prohibition of Nuclear Weapons is an important addition to the corpus of international law. The commentary includes an introduction to nuclear weapons and the international legal framework prior to the treaty's adoption at the United Nations. It then analyses the treaty itself article by article, looking at issues of state responsibility and attribution, jurisdiction, and compatibility with membership of military alliances.