The analogy between the chemistry of molecular transition metal clusters and the processes of chemisorption and catalysis at metal surfaces (the Cluster Surface analogy) has for a number of years provided an interplay between experimental and theoretical inorganic and physical chemists. This collaborative approach has borne fruit in the use of well-defined modes of metal-ligand bonding in discrete molecular clusters, models for metal-ligand binding on surfaces. Some of the key topics discussed in this book are: mechanisms of the fluxional behaviour in clusters in the liquid phase and the connections with diffusion processes on extended surfaces, and the role of metal-metal bond breaking in diffusion; analogies in the structure of chemisorbed species and related ligands on metallic clusters; analogies between benzene surface chemistry on extended metal surfaces and on metal surfaces in molecular cluster compounds with particular reference to structural distortions; and the role of mobile precursors for dissociation of chemisorption on extended metals and on clusters.