Thunderstorms are the birth places of the most energetic natural particle acceleration processes on Earth, and Terrestrial Gamma-ray Flashes (TGFs) are the most explosive manifestation of such processes, capable to deliver 1018 high-energy photons from thunderclouds to outer space in a few tens of microseconds. Together, active and past TGF-observing missions catalog many thousands of events; however, each mission provides only a partial view of the same phenomenon biased by different energy ranges, orbits, selection criteria and instrumental effects. Up to now, no effort has been undertaken to provide a unifying view of the TGF phenomenon by harmonizing the currently available observations in a single, comprehensive population study. This ISSI Team proposal aims at assembling an international team of scientists with the specific goal of providing a quantitative description of the population of TGFs in terms of duration, energy spectrum, fluence distribution and occurrence frequency, based on all current observations. This project shall also identify TGF science requirements not accessible by current or near future missions, as the basis for possible future mission proposals.