The interaction between transition-metal surfaces and simple diatomic molec
ules (CO, NO, H-2, N, and O-2) may lead to the breaking and making of chemi
cal bonds and trigger important surface-catalysed reactions. We discuss the
most common surface interaction and orientation models and consider the el
ectronic structure of the transition metal, and the influence of structure,
bonding and coordination of the diatomic molecule. We emphasize the import
ance of the tilted precursor in the dissociation of diatomic molecules on t
ransition-metal surfaces.