Electric conduction in ovonic materials is analyzed with special attention to chalcogenide glasses used for phase-change memories. A general theory is presented based on plausible microscopic assumptions. Electric field, carrier concentration, and electron temperature along the device, as well as diffusion and Poisson self-consistency, are considered. The effect of different ranges of localized levels in the gap is analyzed. The results account for and interpret all main experimental findings in phase-change memory cells. (C) 2013 Elsevier Ltd. All rights reserved.