Nuclear fallout is residual radioisotope material that is created by the reactions producing a nuclear explosion or nuclear accident. In explosions, it is initially present in the radioactive cloud created by the explosion, and "falls out" of the cloud as it is moved by the atmosphere in the minutes, hours, and days after the explosion. The amount of fallout and its distribution is dependent on several factors, including the overall yield of the weapon, the fission yield of the weapon, the height of burst of the weapon, and meteorological conditions.

Fission weapons and many thermonuclear weapons use a large mass of fissionable fuel (such as uranium or plutonium), so their fallout is primarily fission products, and some unfissioned fuel. Cleaner thermonuclear weapons primarily produce fallout via neutron activation. Salted bombs, not widely developed, are tailored to produce and disperse specific radioisotopes selected for their half-life and radiation type.

Fallout also arises from nuclear accidents, such as those involving nuclear reactors or nuclear waste, typically dispersing fission products in the atmosphere or water systems.

Fallout can have serious human health consequences on both short- and long- term time scales, and can cause radioactive contamination far away from the areas impacted by the more immediate effects of nuclear weapons. Atmospheric and underwater nuclear weapons testing, which widely disperses fallout, was ceased by the United States, Soviet Union, and United Kingdom following the 1963 Partial Nuclear Test Ban Treaty. Underground testing, which can sometimes causes fallout via venting, was largely ceased following the 1996 Comprehensive Nuclear-Test-Ban Treaty. The bomb pulse, the increase in global carbon-14 formed from neutron activation of nitrogen in air, is predicted to dominate long-term effects on humans from nuclear testing, causing ill effects and death in a small fraction of the population for up to 8,000 years.