TPNW - Treaty on the Prohibition of Nuclear Weapons, entered into effect in January 2021, 90 days following the 50th ratification of the Treaty. A substantial majority of the world's countries voted in favour of the Treaty in the United Nations General Assembly. This book presents and discusses the Treaty. Its sister book 'Nuclear Weapons and International Law (3rd edition), NWIL3, could have been updated to include material following the Treaty entry into force but that would have made an already substantial book, even larger. It was decided to issue a much smaller stand-alone book, that could contain all key points in a less expensive book. The chapters of the book provide compact discussions of a number of essential topics. It starts with a history of the journey from the birth of the bomb in the late 1930s-early 1940s to the first signings of TPNW in 2017. This history includes the intimates relationship between the UK and USA starting with British participation in the Manhattan Project right through to today. Another Chapter shows how nuclear weapons have been unlawful under international law since the beginning and that the bombings of Hiroshima and Nagasaki were unlawful. In this sense TPNW can be seen as a codifying treaty given the already unlawfulness of nuclear weapons under several treaties and customary international law; as found by the International Court of Justice who failed to identify any circumstances under which the use of nuclear weapons would undoubtedly be lawful. The nuclear weapons states are not happy about TPNW; they prefer the cosy status quo set up by the 1968 Non-Proliferation Treaty of which they are all in material breach because of their failure to negotiate in good faith for multilateral nuclear disarmament. During the negotiations for TPNW and following its entry into force their main argument was that TPNW would undermine the NPT. This book discusses that, shows how the nuclear weapons states have failed to provide any logical or coherent rationale as to how TPNW could undermine the NPT, and goes on to show the opposite; TPNW complements and reinforces the NPT; they sit side-by-side very comfortably. Implementation of TPNW requires domestic legislation; a 1987 New Zealand Act is presented as an example of how that can be done. The book includes a discussion of how the legal framework in the world would benefit from a Nuclear Weapons Convention. There is a UK case study of its nuclear weapons policy, including considerat