The book examines changes in the properties of steel under the influence of hydrogen absorbed by steel during electrochemical processes, as well as from a gaseous medium under conditions of high temperatures and pressures. The issue of the location and form of the state of hydrogen in steel is covered, modern ideas are given about the role of hydrogen in changing the mechanical characteristics of steel both under short-term loading and under long-term static and cyclic loading; (information is given on the effect of hydrogen on some electrical, magnetic and electrochemical properties of steel, on the role of hydrogen in the process of its destruction, modern theories of hydrogen brittleness are presented).