Metal-metal interactions are very common in dinuclear and polynuclear metal complexes and play an important role in determining their magnetic, electronic and catalytic properties. We are studying metal-metal bonding in dinuclear and polynuclear transition metal complexes with the overall aim to identify and quantify the underlying electronic factors that affect the extent and type of metal-metal interaction such as the identity of the metal ion, it's oxidation state and coordination environment. Density functional methods combined with the broken-symmetry approach to analyzing the magnetic interactions between metal centers, has proven to be a reliable and efficient procedure for calculating the geometries and magnetic properties of such complexes spanning the whole range of metal-metal interactions from weak magnetic coupling through to multiple metal-metal bonding. Using this approach, we have determined the periodic and redox-induced trends in metal-metal bonding for a whole series of dndn (n = 1 - 5) dinuclear complexes as well as dndn-1 (n = 2 - 5) mixed-valence systems. These calculations have established the strong dependence of the metal-metal bonding on key electronic factors such as the unpaired spin-density on the metal centers.