This book is the international edition of the proceedings of IS-Seoul 2011, the Fifth International Symposium on Deformation Characteristics of Geomaterials, held in Seoul, South Korea, in September 2011.The book includes 7 invited lectures, as well as 158 technical papers selected from the 182 submitted. The symposium explored ideas about the complex load-deformation response in geomaterials, including laboratory methods for small and large strains; anisotropy and localization; time-dependent responses in soils; characteristics of treated, unsaturated, and natural geomaterials; applications in field methods; evaluation of field performance in geotechnical structures; and physical and numerical modeling in geomechanics. These topics were grouped under a number of main themes, including experimental investigations from very small strains to beyond failure; behavior, characterization and modeling of various geomaterials; and practical prediction and interpretation of ground response: field observation and case histories. Both the symposium and this book represent an important contribution to the exchange of advanced knowledge and ideas in geotechnical engineering and promote partnership among participants worldwide.