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.