Common physical processes are identified in various types of natural and artificially triggered lightning flashes, both in summer and winter storms. By applying an electrostatic model of bidirectional, uncharged and monopolar, charged leaders, the main physical principles are defined for interpretation of common lightning processes. These principles focus on lightning initiation, charger on the leader, the leader's electrical potential, the electrical breakdown at the leader tip, leader branching and current cutoff, the occurrence of recoil streamers, and the conclusion of lightning propagation. The bidirectional, uncharged leader model is compared, on both physical grounds and by analyzing the electric field changes, with the conventional model of a unidirectional, uniformly charged leader which originates from a space charge source. These two models are also tested against experimental data obtained with a VHF mapping system and in situ measurements of electric fields in storms on the heights of lightning origins in cloud-to-ground flashes. ¿ American Geophysical Union 1992