During recent years, studies of the activation of hydrogen by soluble catalysts have been intensely pursued. The scope of this chapter has been limited to those catalysts utilizing hydrogen gas in the reduction of organic substrates. The first hydrogenation of an organic molecule using a soluble catalyst rather than classical divided metal catalysts was reported in 1938. Other catalyst studies followed. Homogeneous and heterogenous catalysts employ a similar range of metals, but soluble complex catalysts are uniform and therefore show more clearly defined activity and selectivity. Variations of ligands in a soluble catalyst, resulting in arrange of properties, may be likened to poisoning of metal surfaces; but use of different ligands in theory makes possible precise control of a range of properties in soluble catalysts. Many developments are to come and with them the increasing utility of soluble catalyst in general organic synthesis.