Book excerpt: The cluster which is an ensemble of two thousands of atoms or molecules, has emerged as a completely new class of materials at the frontier of materials science. The frontier of cluster science extends so rapidly that the map of the science is renewed day by day. In order to provide basic knowledge and recent information on this growing field, 14 world-renowned scientists who are actively involved experimentally and theoretically in cluster science have written this book, which is concise, comprehensive, suitable for students at both the undergraduate and the graduate level, as well as people who work outside cluster science.