The world's largest economies have set clear development plans for hydrogen energy. From an Economy, Energy, and Environment (3E) point of view, hydrogen energy can be considered an ideal technology for enabling the energy transition from fossil fuels, restructuring energy systems, securing national energy sources, accelerating carbon neutralization, and driving the development of technologies and industry. Green hydrogen production by water electrolysis is the key for hydrogen energy, and this book offers urgently needed guidance on the most important scientific fundamentals and practical applied technologies in this field. This book: • Details materials, electrochemistry, and mechanics. • Covers ALK, PEM, AEM, and SOEC water electrolysis, including fundamentals and applications. • Addresses trends, opportunities, and challenges. This comprehensive reference is aimed at engineers and scientists working on renewable and alternative energy to meet global energy demands and climate action goals. The Open Access version of this book, available at http://www.taylorfrancis.com, has been made available under a Creative Commons Attribution-Non Commercial-No Derivatives (CC-BY-NC-ND) 4.0 license.