The UN Treaty on the Prohibition of Nuclear Weapons will finally come into force after the 50th country (Honduras) ratified it over the weekend. The treaty will make the development, testing, stockpiling and use of nuclear weapons illegal for those countries that have signed it. This is an extraordinary achievement for those who have suffered the most from these weapons — including the hibakusha (survivors) of the bombings of Hiroshima and Nagasaki and the islanders who lived through nuclear weapons testing in the Pacific. Since 1956, the hibakusha in Japan, South Korea, Brazil and elsewhere have been some of the most strident campaigners against the use of these weapons. Among them is a group of Japanese Catholics from Nagasaki whom I interviewed as part of my research collecting the oral histories of atomic bomb survivors.