Proton exchange membrane fuel cell (PEMFC) performance degrades when carbon monoxide (CO) is present in the fuel gas; this is referred to as CO poisoning. This paper investigates CO poisoning of PEMFCs by reviewing work on the electrochemistry of CO and hydrogen, the experimental performance of PEMFCs exhibiting CO poisoning, methods to mitigate CO poisoning and theoretical models of CO poisoning. It is found that CO poisons the anode reaction through preferentially adsorbing to the platinum surface and blocking active sites, and that the CO poisoning e!ect is slow and reversible. There exist three methods to mitigate the e!ect of CO poisoning: (i) the use of a platinum alloy catalyst, (ii) higher cell operating temperature and (iii) introduction of oxygen into the fuel gas #ow. Of these three methods, the third is the most practical. There are several models available in the literature for the e!ect of CO poisoning on a PEMFC and from the modeling e!orts, it is clear that small CO oxidation rates can result in much increased performance of the anode. However, none of the existing models have considered the e!ect of transport phenomena in a cell, nor the e!ect of oxygen crossover from the cathode, which may be a signi"cant contributor to CO tolerance in a PEMFC. In addition, there is a lack of data for CO oxidation and adsorption at low temperatures, which is needed for detailed modeling of CO poisoning in PEMFCs.