This book systematically elucidates the latest breakthroughs in Grain Boundary Engineering (GBE) technology for addressing the critical industry challenge of intergranular stress corrosion cracking in key components of nuclear power plants. Focusing on austenitic stainless steels and nickel-based alloys, it provides an in-depth exploration of how precise grain boundary design and control fundamentally enhance material service performance and longevity under extreme operating conditions. A distinctive feature of this work is its application of advanced 3D microstructural characterization, which clearly reveal the mechanisms behind grain boundary network evolution and performance optimization. Furthermore, it introduces innovative processing methodologies applicable to large-scale engineering components, effectively bridging the gap between GBE theory and industrial practice. This volume serves as an essential reference for researchers and engineers in materials science and nuclear engineering, as well as for graduate students and faculty in related disciplines.