The article presents a review of the current state of the theory of avalanche-streamer processes, i.e., processes occurring during the development of discharges in dense gases. The principal attention is devoted to description of the physical picture of the phenomena. A systematic presentation is given of the theory of the main stages of formation of a discharge: the theory of electron avalanches, the theory of a selfmaintaining discharge, the theory of the avalanche-streamer transition, and the theory of streamers. The question of breakdown of long spark gaps is discussed briefly.