Austenitic alloys, including 690 and other nickel-based alloys, 304, 316, 347 and other stainless steels, and austenitic duplex stainless steels such as Z3CN20.09M, demonstrate superior corrosion resistance and mechanical properties. These alloys serve extensively as pressurized structural materials in nuclear and thermal power plants, where they operate under extreme conditions (high temperatures, corrosion, stress) during long-term service. The primary failure modes observed are corrosion and stress corrosion along grain boundaries. Therefore, enhancing intergranular damage resistance through grain boundary design and control remains a critical focus of materials research. Grain boundary engineering (GBE) technology offers a promising approach. This chapter examines the implementation of GBE in austenitic alloys. Multiple thermo-mechanical processing procedures have been established following GBE principles to facilitate various materials processing. The research examined how initial microstructure, pre-deformation amount, and annealing parameters affect the GBE-ed microstructure. The distinctive microstructural characteristics of GBE-treated materials were identified. Research demonstrates that GBE treatment produces a characteristic microstructure, characterized by high proportions of low-∑ coincidence site lattice (CSL) grain boundaries, large twin-related domains (or named grain-clusters), and extensive long-range twin boundary chains. This modified microstructure shows potential for improving the intergranular degradation resistance of these materials.