It is well established that small amounts of internal hydrogen produce embrittlement and cracking of ferritic and high-strength martensitic steels. Hydrogen has a large solubility in molten steel, and copious quantities of the gas can be absorbed in the liquid during the steel-making process and retained over and above the equilibrium solubility during subsequent solidification. The hydrogen exerts a high internal pressure and, aided by transformation and thermal contraction stresses, produces defects in ingots and castings and hairline cracking in large forgings and heavy-section steel plates. These deleterious effects have long been recognized, and it is now the practice to remove the hydrogen by vacuum melting and pouring, cooling very slowly, and/or holding of semi-finished or finished products for prolonged periods at high temperatures to allow the hydrogen to diffuse out of the steel. However, embrittlement and cracking may also be produced as a result of hydrogen pick-up during secondary fabrication (acid pickling, welding, cadmium and zinc plating operations, cathodic protection procedures, etc.) and service [electrolytic reactions, corrosion in acids and aqueous solutions, and exposure to high-pressure hydrogen gas at ambient and elevated temperatures and to sour (H2S-containing) gases and liquids].