This book is devoted to innovative structural materials for multi-materialization. It is based on results of a 10-year national project, The Innovative Structural Materials Research and Development Project, which was carried out in Japan, aimed at reducing the weight of materials (steel, aluminum alloys, magnesium alloys, titanium alloys, thermoplastic CFRP, carbon fiber) and components used in transportation equipment such as automobiles. In this project, collaborative research in a total of nine fields including materials, joining, and structural designing was also carried out in order to realize multi-materials.