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. This book is compiled with the aim of handing down the technical and academic results obtained through these research and development activities to the next generation of researchers and students. This book enables material engineers and researchers in the field of materials related to transportation equipment, graduate students in various technical fields, and engineers and researchers belonging to material users to grasp the full picture of material development and multi-materials technologies. For the understanding of engineers and researchers who will work on multi-materials, this book explains the current state of technology and science in each field and explains the innovative results obtained by research in each field.