The main obstacles to greater commercialization of polymer electrolyte fuelcells are mostly related to the low-proton conductivity at low-relativehumidity of the known ionomeric membranes, to their high methanol permeabilityand poor mechanical properties above ∼130°C. A possible solution forthese problems has been found in the development of composite membranes, whereparticles of suitable fillers are dispersed in the ionomer matrix. Thepreparation methods for obtaining composite membranes are described, and recentwork dealing with composite ionomeric membranes containing silica,heteropolyacids, layered metal phosphates, and phosphonates is reviewed.Finally, new strategies for the preparation of nano-composite membranes and forthe filling of porous polymeric membranes with highly conductive zirconiumphosphonates are described. The expected influence of size and orientation ofthese particles on membrane properties, such as conductivity and permeabilityto methanol, is also discussed.