This volume collects a a number of contributions on spontaneous symmetry breaking. Current studies in this general field are going ahead at a full speed. The book present review chapters which give an overview on the major break throughs of recent years. It covers a number of different physical settings which are introduced when a nonlinearity is added to the underlying symmetric problems and its strength exceeds a certain critical value. The corresponding loss of symmetry, called spontaneous symmetry breaking, alias self-trapping into asymmetric states is extensively discussed in this book. The book presents both active theoretical studies of spontaneous symmetry breaking effects as well as experimental findings, chiefly for Bose-Einstein-Condensates with the self-repulsive nonlinearity, and also for photorefractive media in optics.