Almost all main group and subgroup metals are able to form metal-metal bonds. The bond order ranges from weak interaction to a quadruple bond, and the degree of aggregation from a dinuclear entity to a three-dimensional network. In spite of numerous physicochemical studies, not all aspects of the metal-metal bond are understood. The ability of metal-metal linked polynuclear complexes to serve as a reservoir for missing or excess electrons enables them to react both with nucleophilic or reducing reagents and with electrophilic or oxidizing reagents. The intermediate position occupied by clusters between simple complexes and the bulk metal is of theoretical and practical significance.