Conductivity spectroscopy spans more than 17 decades on the frequency scale, ranging from about 100 mu Hz up to more than 10 THz. It provides a time-resolved view of the hopping dynamics of the mobile ions in materials with disordered structures, from the individual hop to macroscopic transport, including correlation phenomena. The characteristic dispersion of the conductivity found in all glassy and structurally disordered crystalline electrolytes is briefly discussed and compared with the Debye-Huckel-Falkenhagen effect in strong dilute aqueous electrolytes. An introduction is given into the experimental techniques employed in measuring complex conductivities in the radio and microwave frequency regimes. The connection between frequency-dependent conductivities and time-dependent functions is outlined. Eventually, we present a set of rules for the development of the ion dynamics with time. The rules are expressed in terms of coupled equations, which constitute the MIGRATION concept. The meaning of the acronym is MIsmatch Generated Relaxation for the Accommodation and Transport of IONs. Model conductivity spectra derived from the MIGRATION concept compare favourably with experimental ones. (c) 2005 Elsevier B.V. All rights reserved.