Enriching uranium means increasing the percentage of uranium-235 (U-235) in natural uranium, which is composed of three major isotopes: uranium-238, uranium-235, and uranium-234. Uranium enrichment is a critical component for both civil nuclear power generation and military nuclear weapons. The nuclear fuel used in a nuclear reactor needs to have a higher concentration of the U-235 isotope than that which exists in natural uranium ore. U-235 when concentrated (or "enriched") is fissionable in light-water reactors, which is the most common reactor design in the USA. During fission, the nucleus of the atom splits apart producing both heat and extra neutrons. Under controlled conditions, these extra neutrons can cause additional, nearby atoms to fission and a nuclear reaction can be sustained.

Enriched uranium is produced through the process of isotope separation, which requires uranium to be in a gaseous form at relatively low temperature. The most common method of uranium enrichment is gas diffusion, which was the first commercial process used in the United States to enrich uranium. However, this method has been replaced by second-generation technology, which requires far less electric power to produce equivalent amounts of separated uranium. Another method is gas centrifugation, which involves spinning uranium hexafluoride gas at high speeds in a centrifuge to separate the isotopes. Laser separation technology is also under development for possible use to enrich uranium.

After enrichment, the remaining uranium-238 is known as depleted uranium (DU), which is considerably less radioactive than even natural uranium, though still very dense. Depleted uranium is used as a radiation shielding material and for armor-penetrating weapons.