High temperature electrolysis (HTE), which is the highly efficient electrolysis of steam at high temperature and utilises the heat and electrical power supplied simultaneously by advanced nuclear reactor, provides a very promising way for massive production of hydrogen in the future. Operations at high temperatures reduce the electrical energy requirement for the electrolysis and also increase the thermal efficiency of the power-generating cycle. This book provides an overview of HTE technology including its key characteristics and challenges of SOEC development, as well as an introduction of the mechanism and system configuration of hydrogen production through HTE and detailed descriptions of thermo-hydrogen transformation efficiency.