Proton exchange membrane (PEM) fuel cells hold the promise of environmentallyfriendly power generation due to their low/zero emissions in comparison withinternal combustion engines. PEM fuel cells also have several other advantagesover conventional energy converting devices, including both high efficiency andpower density, which make them unique across a wide range of portable,stationary, and transportation power applications. However, several challengesremain, including cost and reliability/durability. Intensive R&D is therefore stillneeded to address these challenges in order to achieve sustainablecommercialization. In this ongoing R&D, fuel cell testing and diagnostics play acritical role in material characterization, performance optimization, designvalidation, and fundamental understanding for further development. Among thetesting and diagnostic tools used in fuel cell R&D, AC impedance spectroscopy (orelectrochemical impedance spectroscopy, EIS) is regarded by scientists andengineers as a powerful technique.