The physical and chemical properties of a new class of lithium conducting polymer electrolytes formed by dispersing ceramic powders at the nanoscale particle size into a poly(ethylenoxide) (PEO)- lithium salt, LiX complexes, are reported and discussed. These true solid-state PEO-LiX nanocomposite polymer electrolytes have in the 30-80 degrees C range an excellent mechanical stability (due to the network of the ceramic fillers into the polymer bulk) and high ionic conductivity (promoted by the high surface area of the dispersed fillers). These important and unique properties are accompanied by a wide electrochemical stability and by a good compatibility with the lithium electrode (assured by the absence of any liquids and by the interfacial stabilizing action of the dispersed filler), all this making these nanocomposite electrolytes of definite interest for the development of advanced rechargeable lithium batteries.