The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is a multilateral treaty to ban nuclear weapons test explosions and any other nuclear explosions, for both civilian and military purposes, in all environments. It was adopted by the United Nations General Assembly on 10 September 1996, but has not entered into force, as it has not been ratified by all the required states. The treaty will establish the Comprehensive Nuclear-Test-Ban Treaty Organization to oversee monitoring, which is currently done by a Preparatory Commission.
Despite not being in force, the CTBT has established a global norm against nuclear testing, reinforced by nuclear testing moratoria declared by the five nuclear-weapon states recognized under the Treaty on the Non-Proliferation of Nuclear Weapons (NPT): China, France, Russia, the United Kingdom, and the United States. Since the treaty's opening for signature in 1996, only sixteen nuclear tests have been conducted, all by non-signatory states: Pokhran-II by India and Chagai-I by Pakistan in May 1998, and six tests by North Korea from 2006 to 2017. Over 2,000 nuclear tests were conducted between 1945 and 1996, primarily by the United States and the Soviet Union.
The CTBT's International Monitoring System is a global nuclear detonation detection system. It is intended to comprise 321 monitoring stations and 16 laboratories across 89 countries, of which ~90% are already operational. The stations collect one of seismic, infrasonic, hydroacoustic, and radionuclide signatures of nuclear explosions. The IMS successfully characterized all six North Korean nuclear tests, and also collects large datasets valuable to scientists for astrophysics, biology, and geology and more.
As of 2026, 188 states have signed and 179 states have ratified the treaty. The treaty designates a "Annex 2" list of states, which must all ratify the CTBT for it to enter into force. These states possessed any nuclear reactors at the time. Nine Annex 2 states have yet to ratify: the six signatories China, Egypt, Iran, Israel, Russia, and the United States, and the three non-signatories India, Pakistan, and North Korea. Israel is also widely believed to possess nuclear weapons, and the nuclear program of Iran is heavily scrutinized internationally.
The CTBT was negotiated in 1995-96 in the Conference on Disarmament with a final phase of negotiations in the UN General Assembly. It built on two major previous treaties, the 1963 Partial Nuclear Test Ban Treaty and the 1968 NPT, as well as the 1974 Threshold Test Ban Treaty between the US and Soviet Union limiting the yields of their underground tests to less than 150 kilotons of TNT.
While the United States was the first nation to sign the CTBT, the United States Senate rejected ratification in 1999. In 2023, in the context of the Russo-Ukrainian war, Russia withdrew its ratification of the CTBT. Non-ratifying states typically explain their reasoning via the non-ratification of a rival, or other regional stability issues.
The movement for international control of nuclear weapons began in 1945, with a call from Canada and the United Kingdom for a conference on the subject. In June 1946, Bernard Baruch, an emissary of President Harry S. Truman, proposed the Baruch Plan before the United Nations Atomic Energy Commission, which called for an international system of controls on the production of atomic energy. The plan, which would serve as the basis for U.S. nuclear policy into the 1950s, was rejected by the Soviet Union as a US ploy to cement its nuclear dominance.
Between the Trinity nuclear test of 16 July 1945 and the signing of the Partial Test Ban Treaty (PTBT) on 5 August 1963, 499 nuclear tests were conducted. Much of the impetus for the PTBT, the precursor to the CTBT, was rising public concern surrounding the size and resulting nuclear fallout from underwater and atmospheric nuclear tests, particularly tests of powerful thermonuclear weapons (hydrogen bombs). The Castle Bravo test of 1 March 1954, in particular, attracted significant attention as the detonation resulted in fallout that spread over inhabited areas and sickened a group of Japanese fishermen. Between 1945 and 1963, the US conducted 215 atmospheric tests, the Soviet Union conducted 219, the UK conducted 21, and France conducted 4.
In 1954, following the Castle Bravo test, Prime Minister Jawaharlal Nehru of India issued the first appeal for a "standstill agreement" on testing, which was soon echoed by the British Labour Party. Negotiations on a comprehensive test ban, primarily involving the US, UK, and the Soviet Union, began in 1955 following a proposal by Soviet first secretary Nikita Khrushchev. Of primary concern throughout the negotiations, which would stretch—with some interruptions—to July 1963, was the system of verifying compliance with the test ban and detecting illicit tests. On the Western side, there were concerns that the Soviet Union would be able to circumvent any test ban and secretly leap ahead in the nuclear arms race. These fears were amplified following the US Rainier shot of 19 September 1957, which was the first contained underground test of a nuclear weapon. Though the US held a significant advantage in underground testing capabilities, there was worry that the Soviet Union would be able to covertly conduct underground tests during a test ban, as underground detonations were more challenging to detect than above-ground tests. On the Soviet side, conversely, the on-site compliance inspections demanded by the US and UK were seen as amounting to espionage. Disagreement over verification would lead to the Anglo-American and Soviet negotiators abandoning a comprehensive test ban (i.e., a ban on all tests, including those underground) in favor of a partial ban, which would be finalized on 25 July 1963. The PTBT, joined by 123 states following the original three parties, banned detonations for military and civilian purposes underwater, in the atmosphere, and outer space.
The PTBT had mixed results. On the one hand, enactment of the treaty was followed by a substantial drop in the atmospheric concentration of radioactive particles. On the other hand, nuclear proliferation was not halted entirely (though it may have been slowed) and nuclear testing continued at a rapid clip. Compared to the 499 tests from 1945 to the signing of the PTBT, 436 tests were conducted over the ten years following the PTBT. Furthermore, US and Soviet underground testing continued "venting" radioactive gas into the atmosphere. Additionally, though underground testing was generally safer than above-ground testing, underground tests continued to risk the leaking of radionuclides, including plutonium, into the ground. From 1964 through 1996, the year of the CTBT's adoption, an estimated 1,377 underground nuclear tests were conducted. The final non-underground (atmospheric or underwater) test was conducted by China in 1980.
The PTBT has been seen as a step towards the Nuclear Non-proliferation Treaty (NPT) of 1968, which directly referenced the PTBT. Under the NPT, non-nuclear weapon states were prohibited from possessing, manufacturing, and acquiring nuclear weapons or other nuclear explosive devices. All signatories, including nuclear weapon states, were committed to the goal of total nuclear disarmament. However, India, Pakistan, and Israel have declined to sign the NPT on the grounds that such a treaty is fundamentally discriminatory as it places limitations on states that do not have nuclear weapons while making no efforts to curb weapons development by declared nuclear weapons states.
In 1974, a step towards a comprehensive test ban was made with the Threshold Test Ban Treaty (TTBT), ratified by the US and Soviet Union, which banned underground tests with yields above 150 kilotons. In April 1976, the two states reached agreement on the Peaceful Nuclear Explosions Treaty (PNET), which concerns nuclear detonations outside the weapons sites discussed in the TTBT. As in the TTBT, the US and Soviet Union agreed to bar peaceful nuclear explosions (PNEs) at these other locations with yields above 150 kilotons, as well as group explosions with total yields over 1,500 kilotons. To verify compliance, the PNET requires that states rely on national technical means of verification, share information on explosions, and grant on-site access to counterparties. The TTBT and PNET entered into force on 11 December 1990.