"Only 20 years after the discovery of Cosmic Gamma-ray Bursts from the universe another completely unknown phenomenon involving gamma-rays was discovered by coincidence the BATSE instrument on the Compton Gamma-Ray Observatory. Short-lived (~1 ms) and very energetic photon emissions (>1 MeV and later: >40 MeV) were found to originate from the Earth’s atmosphere and were named Terrestrial Gamma Flashes (TGFs). These flashes are the most energetic natural photon phenomenon that is known to exist on Earth, in which also anti-matter is produced. Based on the few datasets available to date we believe that TGFs are related to electric discharges in thunderstorm systems and that electrons accelerated to relativistic energies are involved to produce bremsstrahlung of such high energies. However, it is not known how frequent TGFs are, the altitude range and the spatial extent of their source region, to what kind of thunderstorms and lightning they are related or the implications of relativistic electrons and positrons ejected into space. There is no consensus on how TGFs are produced. All these questions need to be answered before we understand how important they are and how they may affect the Earth’s electrical circuit and atmosphere.The goal of the TGF-MEPPA project is to attack these questions by combining modelling of electron acceleration in thunderstorm electric fields, X- and gamma-ray production and propagation, lightning development with unprecedented measurements of TGFs from three different altitudes: 350 km, 30 km and 20 km to obtain the most comprehensive and detailed dataset needed to make significant advances in the TGF research. I will also perform electric discharge experiments in the laboratory. The goal is to establish a consistent model for the TGF-production and answer the question ‘How common are TGFs?’ to determine their implications for the Earth’s electrical circuit, atmosphere and outer space."