The idea for this book originated from the Joint Discussion 10 of the XXIV IAU General Assembly held in Manchester (England) in the summer of 2000. The extremely successful session on mergers in clusters of galaxies persuaded the publisher of this book to have a volume on such a topic. Clusters of galaxies are by now recognized to be not simple relaxed structures, but rather they are evolving via merging processes in a hierarchical fashion from poor groups to rich clusters. Much progress has been made in recent years in the observations of the signatures of merging processes at many wavelengths
