The dynamics of mobile ions in materials with disordered structures constitute a challengingly complicated many-particle process. Time-resolved information on the dynamics is obtained from spectra of the complex ionic conductivity, which span more than 17 decades on the frequency scale. Ionic crystals with structural disorder as well as ion-conducting glasses, melts and polymers are found to display similar characteristics of their complex conductivity spectra. The features observed in the spectra led us to formulate a set of simple rules for the ion dynamics. The rules may be written as rate equations, forming the so-called MIGRATION concept. The acronym stands for MIsmatch Generated Relaxation for the Accommodation and Transport of IONs. Ionic movements that remain localised are described by a modified version of the MIGRATION concept, which accounts for the NCL (Nearly Constant Loss) behaviour. In general, all aspects of the experimental conductivity spectra of disordered ionic materials are well reproduced by model spectra comprising MIGRATION-type, NCL-type and vibrational components. (c) 2006 Elsevier B.V. All rights reserved.