A Geiger counter, also called a Geiger–Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a Geiger–Müller tube. Each particle detected produces a pulse of current, but the Geiger counter cannot distinguish the energy of the source particles. Invented in 1908, Geiger counters remain popular instruments used for measurements in health, physics, industry, geology and other fields, because they can be made with simple electronic circuits.

Geiger counters are used to detect ionizing radiation, usually beta particles and gamma rays, but certain models can detect alpha particles. An inert gas-filled tube (usually helium, neon or argon with halogens added) briefly conducts electricity when a particle or photon of radiation makes the gas conductive. The tube amplifies this conduction by a cascade effect and outputs a current pulse, which is then often displayed by a needle or lamp and/or audible clicks.