This book provides a detailed description of hydrogen production through water electrolysis. It starts with the theoretical description of the chemical, thermodynamic, and kinetic issues related to the electrolysis of water. The main available technologies and the ones under development are detailed from a technical and a scientific point of view. At the end of the book Dr. Cavaliere describes the main hydrogen applications and their contribution to the grand energy transition that is expected by the middle of the century. The book also examines the economic issues related to the transition toward the hydrogen society.