the study of clusters is one of the most exciting topics in the rapidly developing field of nanostructured materials. as discussed in this book, nanometer-sized metal particles can be obtained not only by evaporation methods, producing atomic or molecular beams, but also by the chemical synthesis of metal cluster compounds. these have a well-defined stoichiometry and are composed of metal clusters of a given, homogeneous size, each cluster being surrounded by a shell of ligand molecules. accordingly, the compounds provide excellent model systems for macroscopic assemblies of small metal particles embedded in a dielectric solid. the underlying physical properties are described in terms of a three-dimensional matrix of mutually separated quantum wells. in going from one compound to another, the size and separation of the quantum wells may be varied. in this way, one may study such fundamental properties as the size-induced transition from metallic to molecular behaviour. at the same time, the electronic level structure may be changed in a controlled way, which should confer tunable optical, electrical or magnetic properties. this book summarizes physical experiments performed so far on this challenging new class of materials, as well as the basic aspects of their chemical synthesis. for physicists, chemists and materials scientists with an interest in metal-cluster compounds and their physical properties.