This book is designed to help for developing the general understanding of all the knowledge essential to ensure the reactor structural integrity. For this purpose, it not only explains basic knowledge of nuclear reactor structure engineering but also allows readers to learn how to systematize this knowledge as they study structural standards. This book first defines structural integrity and explains the concept of structural standards that is used for ensuring integrity, and then describes strength of materials (or mechanics of materials), which is vital as a means of designing components’ dimensions and other elements, and the finite element analysis, which has been developed on the basis of strength of materials and has been going mainstream in recent years. Subsequently, the book explains how components are designed, fabricated and examined in practice. In particular, with respect to design, the book places emphasis on what should be considered insizing of components, and provides a description of practical activities of sizing and other elements.