Serious problems can arise when these alloys come into contact with hydrogen-containing environments; the harmful effects of hydrogen in various titanium alloys alter their microstructure and properties, making them susceptible to a well-known problematic phenomenon presented in this chapter. Hydride formation and the consequences on the material embrittlement are discussed. Hydrogen solubility, kinetics, and diffusion in different titanium structures are detailed. The effect of hydrogen on the fatigue and crack propagation behavior is discussed. The effect of hydrogen at high temperatures on the titanium alloy structures and mechanical properties is shown.