Gamma rays ( denoted as γ ) are a form of electromagnetic radiation produced by sub-atomic particle interactions, such as electron-positron annihilation or radioactive decay. They have the highest frequency (above 1019 Hz) and energy (above 100 keV), and also the shortest wavelength (below about 10 picometers), in the electromagnetic spectrum. Gamma rays were discovered by Paul Villard, a French chemist and physicist, in 1900, while studying uranium.

Hard X-rays overlap the range of "long"-wavelength (lower energy) gamma rays. The distinction between the two terms, however, depends on the source of the radiation, not its wavelength; X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei or matter-antimatter annihilation.

As a form of ionizing radiation, gamma rays can cause serious damage when absorbed by living tissue, and they are therefore a health hazard.

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