Electromagnetic interference (EMI) poses significant challenges in high voltage (HV) transmission systems by degrading the performance of protection devices, communication networks, and control equipment. With the increasing complexity of modern power systems—driven by smart grid integration, power electronic interfaces, and renewable energy penetration—the susceptibility to EMI has intensified. This review provides a comprehensive analysis of EMI sources, propagation mechanisms, effects on critical infrastructure, measurement and monitoring techniques, and mitigation strategies. It explores both conducted and radiated interference phenomena and highlights the role of international standards in establishing electromagnetic compatibility (EMC). Furthermore, it outlines real-world applications and presents current research trends in modeling, simulation, and AI-driven EMI diagnostics. The paper concludes by identifying future directions aimed at improving EMI resilience in evolving HV transmission networks. By synthesizing current literature and practice, this review offers insights into robust EMI management, contributing to the safe, reliable, and sustainable operation of electrical power systems