High-strength steels, in particular, can experience a loss of ductility and strength when exposed to hydrogen gas at high pressures, a phenomenon known as hydrogen embrittlement (HE). Numerous mechanisms have been proposed to explain macroscopic brittle and retarded HE, drawing on microplasticity and atomistic theories. They are described in this chapter. Trapping and diffusion in high-strength steels structure is described. Fatigue and crack propagation behavior of high-strength steels in hydrogen environment are discussed.