The Geiger counter is an instrument used for measuring ionizing radiation used widely in such applications as radiation dosimetry, radiological protection, experimental physics and the nuclear industry.

It detects ionizing radiation such as alpha particles, beta particles and gamma rays using the ionization effect produced in a Geiger–Müller tube; which gives its name to the instrument. In wide and prominent use as a hand-held radiation survey instrument, it is perhaps one of the world's best-known radiation detection instruments.