Commercialization of the proton exchange membrane fuel cell, an efficient energy-conversion device, requires additional gains in system lifetime. Contamination represents a key degradation mode. Its status is summarized and analyzed to identify research needs. Contaminant sources include ambient air, system components located upstream of the fuel cell stack, and fuel and coolant loops. The number of reported contaminants was conservatively estimated at 97, but many contaminant compositions are still unclear and many gaps remain to be explored, including airstream system components and coolant and fuel streams. For the latter cases, contaminants may reach the cathode compartment by diffusion through the membrane or as a result of seal or bipolar plate failure, thus representing potential interaction sources. In view of this large potential inventory of contaminants, recommendations were made to accelerate studies, including the addition of identification tests performed by material developers, development of standard tests, and definition of an exposure scale for ranking purposes. Because anions are excluded from the membrane in contact with weak solutions (Donnan exclusion), mechanisms involving anions need to be reevaluated. Contaminant mechanisms were synthesized, resulting in only eight separate cases. This situation favors the development of two key simple mathematical models addressing kinetic and ohmic performance losses that are expected to positively impact the development of test plans, data analysis, model parameter extraction, contaminant classification (use of apparent rate constants), and hypothetical scenario evaluation. Many mitigation strategies were recorded (41) and were downselected by elimination of untimely material-based solutions. The remaining strategies were grouped into three generic approaches requiring further quantitative evaluation and optimization: cathode compartment wash, cathode potential variations, and manufacturing material and processing specifications.