This critical review presents an overview of the various classes of Li(+) conductors for use as electrolytes in lithium polymer batteries and all-solid state microbatteries. Initially, we recall the main models for ion transport and the structure-transport relationships at the basis of the observed conductivity behaviours. Emphasis is then placed on the physico-chemical and functional parameters relevant for optimal electrolytes preparation, as well as on the techniques of choice for their evaluation. Finally, the state of the art of polymer and ceramic electrolytes is reported, and the most interesting strategies for the future developments are described (121 references).