The Partial Nuclear Test Ban Treaty comes into effect.

The Partial Test Ban Treaty is the abbreviated name of the 1963 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, which prohibited all test detonations of nuclear weapons except for those conducted underground. It is also abbreviated as the Limited Test Ban Treaty and Nuclear Test Ban Treaty, though the latter may also refer to the Comprehensive Nuclear-Test-Ban Treaty, which succeeded the PTBT for ratifying parties.

Negotiations initially focused on a comprehensive ban, but this was abandoned due to technical questions surrounding the detection of underground tests and Soviet concerns over the intrusiveness of proposed verification methods. The impetus for the test ban was provided by rising public anxiety over the magnitude of nuclear tests, particularly tests of new thermonuclear weapons, and the resulting nuclear fallout. A test ban was also seen as a means of slowing nuclear proliferation and the nuclear arms race. Though the PTBT did not halt proliferation or the arms race, its enactment did coincide with a substantial decline in the concentration of radioactive particles in the atmosphere.

The PTBT was signed by the governments of the Soviet Union, United Kingdom, and United States in Moscow on 5 August 1963 before being opened for signature by other countries. The treaty formally went into effect on 10 October 1963. Since then, 123 other states have become party to the treaty. Ten states have signed but not ratified the treaty.