In homogeneous reactions catalyzed by transition metal complexes, the active species are in general mononuclear transition metal complexes and their catalytic activities are controlled by an appropriate choice of metals and ligands, especially phosphine ligands. Recently, homogeneous multimetallic catalysis has been receiving much attention because of the potential for the different metals cooperating during chemical transformations. It is expected that this cooperativity aids in the design of new, more active and selective catalysts and new types of reactions which are inaccessible through monometallic systems. Reviewed here are activation of substrates on metal clusters and various types of reactions catalyzed by metal clusters and mixed metal complexes.