Surprisingly, the triggering mechanism for lightning is not known; thundercloud electric fields do not reach the value required for the conventional breakdown of air.   Some models of the breakdown process involve acceleration of electrons to high energy, predicting X- and gamma radiation from bremsstrahlung.  These emissions have been observed in association with lightning strokes, using platforms in space, on the ground, and in the air.    From orbit, they take the form of millisecond-long Terrestrial Gamma-ray Flashes (TGFs); I will discuss recent work on TGFs , including the discovery that they cause beams of relativistic electrons to be launched into the magnetosphere.