Abstract: High-performance permanent magnets play an indispensable role in manufacturing highly efficient motors and generators, ultimately contributing to the sustainability of our planet. Achieving high coercivity (μ₀H_c) in hard magnetic materials at or above room temperature without relying on heavy rare-earth elements remains a challenge. Effective strategies for augmenting μ₀H_c utilizing low-cost materials involve inter-granular addition and the grain-boundary diffusion (GBD) processes. This review provides a comprehensive overview of selected techniques for developing grain-boundary (GB) modification through a specific focus on the reduction of using heavy rare earth (HRE) elements and utilizing cost-effective non-rare earth and light rare earth (LRE) sources for GBD. To date, the successful application of HRE based metals and compounds, and LRE based alloys as diffusion sources has significantly contributed to μ₀H_c enhancement. Drawing upon global advancements and findings from various research groups, this comprehensive review offers a critical analysis of the fundamentals and progress in GBD processes (GBDPs). Considering the different types of permanent magnets, the GBDPs, based on the diverse diffusion sources and diffusion processes, are thoroughly compared and summarized. This encapsulates the current state of knowledge in the field, providing valuable insights into the ongoing developments in GB modification to improve permanent magnet performance.