Polymer electrolyte membrane fuel cells (PEMFC's) show promise as power sources for high-efficiency light-duty vehicles giving very low emissions. This talk reviews some of the critical interfacial phenomena that must be understood if this promise is to be fully realized. Examples will be given of cases where surface analytical techniques have provided useful insights. Additional cases will be discussed in which extension beyond the capabilities of current surface analytical techniques will be required if critical information is to be obtained. Fuel cells predate the internal combustion engine, but two developments since the 1980's led to the ten-fold increase in power density that generated the current enthusiasm for automotive PEMFC's. First, catalyst makers learned to pack ~3x10@super20@ active platinum sites into a cubic cm of porous catalyst layer through the synthesis of ~50% Pt (by weight) catalysts with >25% dispersion on carbon supports with high electronic conductivity. Second, incorporation into the catalyst layer of ionomer similar to that used in the membrane allowed adequate ionic conductivity to be maintained between these active sites and the electrolyte membrane. While the resulting complex nanostructures must maintain some water content to allow ionic conductivity, excessive retention of product water can prevent access of the gaseous reactants to the catalyst particles. The system contains many interfaces: gas/liquid, gas/ionomer, liquid/ionomer, ionomer/metal, metal/carbon, ionomer/carbon (and perhaps gas/metal), all of which must be carefully controlled. Vacuum surface analytical techniques have yielded major clues as to why Pt-alloy catalysts, the enhanced activity of which is necessary if automotive cost targets are to be met, can be stable in the strongly acidic PEMFC environment. Similar analytical successes on water/ionomer/Pt/carbon interfaces are needed to speed progress toward fuel cells that fully meet automotive requirements.