At the heart of a proton exchange membrane fuel cell (PEMFC) membrane electrode assembly (MEA) are two catalyst layers. These layers play a critical role in defining the performance of the MEA. Without them, the MEA would not function, and the PEMFC would not be an exciting energy-generation technology for the 21st century (Hoogers and Thompsett, 2000). It is the purpose of this chapter to discuss the catalysts and their influence on the performance of the state-of-the-art PEMFC.