A nuclear weapon is a type of explosive weapon that derives its destructive force from nuclear reactions of fusion or fission. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives, and a single weapon is capable of destroying an entire city.

In the history of warfare, only two nuclear weapons have been detonated aggressively — both by the United States of America, during the closing days of World War II. The first was detonated on the morning of 6 August 1945, when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second was detonated three days later when the United States dropped a plutonium implosion-type device code-named "Fat Man" on the city of Nagasaki. These bombings resulted in the immediate deaths of around 120,000 people from injuries sustained from the explosion and acute radiation sickness, and even more deaths over time from long-term effects of radiation. The use of these weapons was and remains controversial. (See Atomic bombings of Hiroshima and Nagasaki for a full discussion.)

Other countries may have nuclear weapons or nuclear weapons programs they do not acknowledge publicly. In particular, Israel has modern airborne delivery systems and appears to have an extensive nuclear program with hundreds of warheads (see Israel and weapons of mass destruction), maintains a policy of "ambiguity" with respect to its actual possession of nuclear weapons. According to some estimates, it possesses as many as 200 nuclear warheads. Iran currently stands accused by the United States of attempting to develop nuclear weapons capabilities, though its government states that its acknowledged nuclear activities, such as uranium enrichment, are for non-weapons purposes. South Africa also secretly developed a small nuclear arsenal, but disassembled it in the early 1990s. (For more information see List of states with nuclear weapons.)

Since the Hiroshima and Nagasaki bombings, nuclear explosives have been detonated on over two thousand occasions for testing purposes and demonstration purposes. The only countries known to have detonated such weapons are (chronologically) the United States, the Soviet Union, the United Kingdom, France, the People's Republic of China, India, Pakistan, and North Korea. Synthetic elements, such as einsteinium and fermium, created by neutron bombardment of uranium and plutonium during thermonuclear explosions, were discovered in the aftermath of the first hydrogen bomb test. Apart from their use as weapons, nuclear explosives have been tested and used for various non-military uses.