Lightning may be initiated via an electron avalanche that may occur when energetic electrons (~1 MeV) are accelerated by thunderstorm electric fields to velocities sufficient to produce new energetic electrons during ionizing collisions with nitrogen or oxygen molecules. For the avalanche to occur, the thunderstorm electric field must exceed a critical value called the breakeven field. At any altitude the breakeven field is substantially less than the field usually thought necessary either for dielectric breakdown or for streamer propagation. We show that 23 electric field soundings through thunderstorms seem to confirm that lightning occurs when the electric field exceeds the breakeven field. The soundings also show that the electric field inside storms tends to be limited to magnitudes less than or equal to the breakeven field. This breakeven mechanism may explain why electric field magnitudes greater than 150 kV m-1 are rarely found inside thunderstorms. It may also help explain the initiation of lightning and other types of discharges that either propagate upward from the tops of thunderstorms or occur above them. ¿ American Geophysical Union 1995