Relativistic effects dominate the chemistry of elements of atomic number greater than about 50: s electron shells are contracted and lowered in energy, while d shells are destabilised; differences between the second and third row Transition Series elements are now only partly ascribable to the 'lanthanide contraction'. Relativistic effects on the chemistry of gold, on the surface structures of metals and on the structure, stability and reactivity of carbonyl and hydrocarbon complexes are surveyed, and their consequences in homogeneous and heterogeneous catalysis, hitherto unrecognised, are explored. Reactivity of organometallic complexes is very largely confined to those containing a metal from Groups 8-10 of the first two Transition Series: complexes of Group 11 metals are too weak, and of the third Transition Series too strong, to be useful. In heterogeneous catalysis, methanol synthesis is observed with metals (Au, Cu, Pd, Pt) not forming stable carbonyls, the other metals of Groups 8-12 giving Fischer-Tropsch products with syngas. In reactions of unsaturated hydrocarbons, all palladium catalysts readily release intermediate alkene products to the fluid phase, but platinum catalysts do not. These and related differences are due to the weaker pi-alkene and pi-allylic intermediates, which form on palladium.