The Gas Electron Multiplier (GEM) is a proven amplification technique for position detection of ionising radiation such as charged particles, photons, X-rays and neutrons, in gas detectors. The GEM is a detector containing a densely pierced polymer foil coated with electrodes on both sides which is able to achieve high amplification gains and performance at low cost, even under harsh conditions. A GEM consists of a thin, metal-clad polymer foil, chemically pierced by a high density of holes. On application of a difference of potential between the two electrodes, electrons released by radiation in the gas on one side of the structure drift into the holes, multiply and transfer to a collection region. This gas detector is extensively used in High Energy Physics.