Fuel cells are an important enabling technology for the hydrogen economy and have the potential to revolutionize the way that we power our world. However, many challenges must be overcome before fuel cell systems can be a competitive alternative for consumers. In light of performance, durability, and cost, this book examines the issues and recent progress on key materials for proton exchange membrane fuel cells (PEMFC). The text reviews these materials in terms of properties and functions in fuel cells, such as temperature limits, catalytic activity, catalyst poisoning, protonic conductivities, chemical stability, degradation, microstructure, and processing.