How lightning is initiated in the relatively low electric fields inside thunderclouds and how it can then propagate for tens of kilometers through virgin air are two of the great unsolved problems in the atmospheric sciences. Until very recently it was believed that lightning was entirely a conventional discharge, involving only low-energy (a few eV) electrons. This picture changed completely a few years ago with the discovery of intense x-ray emission from both natural cloud-to-ground lightning and rocket-triggered lightning. This energetic emission cannot be produced by a conventional discharge, and so the presence of x-rays strongly implies that runaway breakdown plays a role in lightning processes. During runaway breakdown, electrons are accelerated through air to nearly the speed of light by strong electric fields. These runaway electrons then emit bremsstrahlung x-rays and gamma-rays during collisions with air. Indeed, the x-ray and gamma-ray emission produced by runaway breakdown near the tops of thunderstorms is bright enough to be seen from outer space, 600 km away. As a result, the physics used for decades to describe thunderstorm electrification and lightning discharges is incomplete and needs to be revisited.