This article examines hydrogen as a valuable alloying element, changing the structure and properties of metallic materials strongly and unusually. An important feature of hydrogen is its anomalously high diffusion mobility in crystalline phases at low temperature. This also opens up possibilities for diffusion-reversible alloying which is that after the action of hydrogen on properties and structure it may be removed from alloys at temperatures below the recrystallization temperature, something that cannot be accomplished by means of any other chemical element. The effect of hydrogen on structure and phase transformations in iron and iron-carbon alloys is examined as is the effect of hydrogen on the structure and properties of nonferrous alloys. Alloying titanium and its alloys with hydrogen appears to be the most promising for nonferrous alloys. The main consequence of the presence of hydrogen in iron and steels during solidification is occurence of porosity. In white cast iron, the presence of hydrogen leads to an increase in the tendency of austenite and cementite to individual crystallisation.