Polymer electrolytes are ionically conducting solid phases formed by the dissolution of salts in ion-coordinating macromolecules. Such materials may be readily fabricated as films using continuous production plant and are being considered as replacements for conventional low-molecular-weight liquid-based electrolytes for practical electrochemical devices such as power sources, variable-transmission windows and displays. The mechanism for ionic transport in polymer electrolytes is novel and quite distinct from the processes occurring in liquid solutions, molten salts or crystalline solid electrolytes. Ion mobility is associated with local structural relaxations of the polymer, although the relationship is not simple. For certain charge carriers local matrix motion simply provides time-dependent pathways or opportunities for ions to move between suitable low-energy sites; for others a mechanism involving short-range transport of ions temporarily attached to the polymer chain is important. For cations, lability of the ion–solvent bond is a necessary condition for long-range transport. The structures and morphology of polymer electrolytes have been studied using a range of techniques. Crystal structures have been reported for certain stoichiometric complexes, but it has been shown that significant ionic conductivity is restricted to amorphous phases. Ionic motion in the latter has been investigated as a function of such variables as salt type, concentration, temperature and the molecular weight of the host polymer. Research on the polymer electrolyte/electrode interface has also been reported; it has been suggested that the constraints on the reorientation of solvating groups in the double layer leads to distinctive behaviour. Polymer electrolytes are likely to hold out the best prospect for practical all-solid-state electrochemical devices since their ability to deform elastically permits efficient interfaces to be fabricated and maintained during cell cycling where volume changes take place in adjacent phases.