This chapter presents a broad overview of water electrolysis as a pathway for hydrogen production. The basic principles and thermodynamics of electrochemical water splitting are discussed. The roles and requirements of the major components of an electrolytic cell are explained. A brief description of the power characteristics of an electrolytic cell is presented by accounting for the different losses, reaction kinetics of water splitting, and hydrogen generation mechanisms, together with considerations of Faradaic efficiency. The chapter surveys established electrolysis technologies such as alkaline, proton exchange membrane, and solid oxide electrolyzers. The emerging alternative is an anion exchange membrane electrolysis. In addition, other approaches for water splitting are outlined, with an emphasis on photoelectrochemical hydrogen production. It aims at a unified understanding of conventional and emerging electrolysis technologies.