The field of cluster science is currently attracting considerable interest, not only from a fundamental standpoint, but also through its future applications to electronic, optical, magnetic, and other devices. Synthesizing specific clus­ ters as a unit of useful nanostructures or controlling them as an assembly of nanocomposites is one of the ultimate purposes in this field. In order to understand how to synthesize individual clusters and t_o investigate physical properties, chemical reactions, structural stability, response to external fields, aggregation, phase transition, and other aspects of clusters, a great deal of effort has gone into experiment, theory and computer simulation in this area. This is presumably motivated by the fact that a high level of collaboration between theoretical and experimental researchers is particularly important for progress in the field of cluster science. The present book aims to collect together recent advances in this rapidly growing field. The authors are all active researchers, collaborating both ex­ perimentally and theoretically in this field, and most of them have regularly participated in the IMR Workshop, held for three years starting from 1998 at the Institute for Materials Research in Tohoku University. This book is suitable for both theoretical and experimental researchers and also for re­ searchers and graduate students working in related subjects, who wish to overview recent advances in the field.