A survey of non-volatile. highly scalable memory devices which utilize dedicated resistive switching phenomena in nanoscale chalcogenide-based memory cells is presented. We introduce the basic operation principle of the phase change mechanism, the thermochemical mechanism, and the valence change mechanism and we discuss the crucial role of structural defects in the switching processes. We show how this role is determined by the atomic structure of the defects, the electronic defect states, and/or the ion transport properties of the defects. The electronic structure of the systems in different resistance states is described in the light of the chemical bonds involved. While for phase-change alloys the interplay of ionicity and hybridization in the crystalline and in the amorphous phase determine the resistances, the local redox reaction at the site of extended defects, the change in the oxygen stoichiometry, and the resulting change in the occupancy of relevant orbitals play the major role in the thermochemical and the valence change mechanism. Phase transformations are not only discussed for phase-change alloys but also for both other types of switching processes. The switching kinetics as well as the ultimate scalability of switching cells is related to structural defects in the materials. (C) 2010 Elsevier Ltd. All rights reserved.