In August 1993, as the shadow of the Cold War began its slow retreat, the United Nations (UN) Conference on Disarmament decided the time was ripe to negotiate a treaty banning nuclear tests once and for all. The end of superpower competition had led three of the five official nuclear powers – the United States (US), Russia, and Britain – to announce testing moratoriums, and nonnuclear states were eager for a universal ban.1 The biggest potential spoiler was China. A “vocal outsider to the global nuclear order”2 and a “latecomer to the nuclear club,”3 China had historically viewed test ban efforts as “ploys intended to monopolize nuclear weapons and solidify the larger nuclear powers’ advantages.”4

Since July 1945, more than 2,000 nuclear explosive devices have been detonated in tests in more than 60 different locations. The total yield of nuclear tests conducted from 1945 to 1995 is equivalent to 34,000 Hiroshima-size bombs. Only North Korea has test-detonated nuclear weapons this century. While the 1963 Partial Test-Ban Treaty remains in force the 1996 Comprehensive Nuclear-Test-Ban Treaty has not entered into force. The chapter explains why and what customary rules restrain nuclear testing today.