The review provides a comprehensive overview on the major findings regarding ion redistribution at interfaces in oxide systems and its effects on the electrical transport properties. As far as interfaces are concerned, grain boundaries and hence polycrystalline materials are to the fore. Heterocontacts and hence composites are only considered if relevant for the general understanding. Selected examples refer to oxide ceramics but also to composites. As far as the fundamental properties are concerned, major emphasis is laid on the impact on ion conductivity. Purely electronic effects (e.g. interfaces in semiconductors, boundaries in superconductors, or the formation of two-dimensional electron gases at the interface between two insulators) as well as phenomena related with solid/fluid interfaces are not addressed. (C) 2017 Elsevier Ltd. All rights reserved.