A wide variety of systems, properties, concepts, and perspectives lie within the scope of this volume on Metallic Systems: A Quantum Chemist’s Perspective. Of course, there is no single “quantum chemist’s perspective.” To some, a “metallic system” implies typical bulk metallic properties, electrical conductivity, thermal conductivity, and the like. To others, the presence of a single metal atom in a molecule or complex is satisfactory for its classi–cation as a “metallic system.” In between, one –nds the fascinating world of metal clusters with properties that are sometimes importantly similar to those of the bulk metal and sometimes crucially different. Both perspectives have value. Metal clusters have their own special properties, depending, for example, on cluster size, and there is a clear interest in taking advantage of them to tune the performance for a particular application, for example, catalysis. On the other hand, one can use clusters as models for either bulk systems or larger nano-and micro-clusters such as those present in working catalysts. Of course, the quality of the model will depend crucially on the size of the cluster.