Reduction is one of the more important and synthetically useful reactions, and has been extensively studied. Among a wide number of reductive procedures, catalytic hydrogenation constitutes a common practice in chemistry, and numerous, highly active metal systems have been developed. Reactive metal powders have been also obtained by reduction of metal salts with either sodium borohydride or the Willstäter procedure, reduction of metal salts by formaldehyde under alkaline conditions. These systems along with Raney nickel and Urushibara catalysts facilitate numerous hydrogenation processes. A wide variety of nitrogen-based functional groups can be conveniently reduced to the corresponding amine derivatives by using dissolved metals, such as zinc, tin, or iron and an acid, or catalytic hydrogenation. Reduction of alkynes gives pure cis-alkenes especially in the presence of 1,2-diaminoethane, and conjugated dienes can be selectively semihydrogenated. Importantly, hydrogenation is usually performed without hydrogenolysis.