Terrestrial Gamma-ray Flashes (TGFs) have become an unexpectedly rich area of scientific discovery for the Fermi Gamma-ray Space Telescope. The Gamma-ray Burst Monitor (GBM) observed a modest rate of one TGF per month over its first year of operation. Through a sequence of flight software enhancements and new observing modes, GBM now detects two TGFs on-board every week, with many more TGFs uncovered on the ground.
This is a particularly interesting time for TGF science with observations by the Reuven Ramati High Energy Solar Spectroscopic Imager (RHESSI), the Astrorivelatore Gamma a Immagini Leggero (AGILE), and more recently by the Fermi Large Area Telescope (LAT). I will discuss global properties of TGFs observed by these instruments, before focusing on the population of TGFs observed by GBM: their intensity distribution, temporal properties, and the very close relationship between TGFs and associated radio signals detected by the World Wide Lightning Location Network.