A Geiger-Mueller counter consists of a tube filled with an inert gas and an organic vapor. The tube contains electrodes, between which there is an electricalvoltage, but no current flowing. When ionizing radiation passes through the tube, a short, intense pulse of current passes "cascades" from the negative electrode to the positive electrode and is measured or counted. The discharge is stopped, or quenched, by the ionized positive ion cloud after the electrons have been collected by the anode. These are called self-quenching counters. The Geiger-Mueller counter has been supplanted by the halogen counter invented by S. H. Liebson in 1947. This is the most commonly used form of Geiger counter. It differs from the Geiger-Mueller counter in that the discharge mechanism takes advantage of the metastable state of the inert gas atom to ionize the halogen molecule and produces a more efficient discharge which permits it to operate at much lower voltages. It also has a longer life because the halogen ions can recombine unlike the organic vapor which is destroyed by the discharge process. Most detectors include an audio amplifier that produce an audible click on discharge. The number of pulses per second measures the intensity of the radiation field. Some Geiger counters display a dose rate (mRh), but this is subject to error as the instrument does not discriminate between radiation at different energy levels.

Geiger-Muller tubes are of two types: the glass-mantle type and the mica window type. The glass window type will not detect alpha radiation and is usually cheaper. The mica window type will detect alpha radiation but is more fragile.

It was named for Hans Geiger and W. Müller, who invented it in the 1920s.