Polymeric membranes play a critical role during the generation of electricity in hydrogen/air and direct methanol proton-exchange membrane (PEM) fuel cells. The membrane in such devices performs three roles: It physically separates the positive and negative electrodes, prevents mixing of the fuel and oxidant, and provides a conduit for ion (proton) movement between the electrodes.  For hydrogen/air proton-exchange membrane fuel cells, the membrane must exhibit low gas permeability and high proton conductivity.  For a direct liquid methanol PEM fuel cell, the ion-exchange membrane must conduct protons and be a good methanol barrier.  DuPont's Nafion^TM (a perfluoro sulfonic acid polymer) is the membrane of choice for H₂/air PEM fuel cells that operate at a temperature of about 80^oC. For a reformate fuel, it is desirable to operate the fuel cell at a temperature greater than 100^oC (to reduce the effects of CO poisoning). Unfortunately, Nafion looses water and the conductivity is low at such high temperatures, unless the water activity in the fuel gas feed is near unity.  Nafion has also been used in direct methanol fuel cells, but high methanol crossover leads to unwanted fuel losses, poisoning of the air cathode by CO (which is produced when methanol permeates through the membrane and is chemically oxidized at the cathode), and low power output (due to cathode depolarization by methanol). There is considerable research and development around the world to develop new membrane materials for PEM fuel cells with tailored physical and transport properties. Current membrane fuel cell work is focused on:  (i) proton conductors that operate at a temperature of 100-120^oC under low humidity conditions (25% relative humidity) for hydrogen/air fuel cells, (ii) polymeric membranes that conduct protons in the dry state during H₂/air fuel cell operation at high temperatures (160-200^oC), and (iii) proton conducting ion-exchange membranes with low methanol permeability for operating temperatures in the range of 60-140^oC. This talk will present and critique fuel cell membrane research efforts in each of these areas.