Cluster Materials is the fourth volume of the highly successful series Advances in Metal and Semiconductor Clusters. In this volume the focus is on the properties of clusters which determine their potential applications as new materials. Metal and semiconductor clusters have been proposed as precursors for materials or as actual materials since the earliest days of cluster research. In the last few years, a variety of techniques have made it possible to produce clusters in sizes varying from a few atoms up to several thousand atoms. While some measurements are performed in the gas phase on non-isolated clusters, many cluster materials can now be isolated in macroscopic quantities and more convenient studies of their properties become possible. In this volume the authors focus on measurement of optical, electronic, magnetic, chemical and mechanical properties of clusters or of cluster assemblies. All of these properties must fall into acceptable ranges of behaviour before useful materials composed of clusters can be put into practical applications. As evidenced by the various work described here, the realisation of practical products based on cluster materials seems to be approaching rapidly.