Hydrogen Energy and Fuel Cell Primers is a series of concise books that present those coming into this broad and multidisciplinary field the most recent advances in each of its particular topics. Its volumes bring together information that has thus far been scattered in many different sources under one single title, which makes them a useful reference for industry professionals, researchers and graduate students, especially those starting in a new topic of research. These volumes, PEM Water Electrolysis vol 1 and 2, allows these readers to identify the technology gaps for the development of commercially viable PEM electrolysis systems for energy applications. This primer examines the fundamentals of PEM electrolysis and selected research topics that are currently subject of attention by academic and industry community, such as gas cross-over and AST protocols. This lays the foundation for the exploration of the current industrial trends for PEM electrolysis, such as power to gas application, are discussed, with strong focus on the current trends in the application of PEM electrolysis associated with energy storage. These include durability aspects of PEM electrolysis systems and components, accelerated stress test protocols, manufacturing aspects of large-scale electrolyzers and components, gas crossover problems in PEM electrolyzer safety, and challenges associated with high-current density operation of PEM electrolyzers. A technology development matrix for systems and components requirements will also be covered, as well as unconventional PEM water electrolysis systems, such as ozone generators