Klaus-Dieter Kreuer Max-Planck-Institut für Festkörperforschung (Solid State Research) Solid State Research Transport and stability issues of ion conducting separator membranes for fuel cells are critically discussed from a fundamental point of view comprising considerations about structure and dynamics on the molecular scale to the device level. They equally imply polymer-chemical and electro-chemical aspects which are closely related for the considered class of materials. The importance of ion/solvent, residual ion/ion and solvent/polymer interactions for the formation and mobility of charge carriers, selective ionic transport and even as driving forces for nano-scale ordering is emphasized, and it is shown that, apart from simple electrostatics, specific chemical interactions must be considered. Based on the presented rationales, suggestions are being made for the modification of existing and the development of new membrane types not only for fuel cells but also for other electrochemical energy conversion and storage devices such as redox-flow and alkaline ion batteries. K. D. Kreuer Chemistry of Materials 25th anniversary issue 2014