Helicopters are extensively utilized in both military and civilian sectors due to their unique flight capabilities. However, their operational safety is constrained by complex flight envelopes and inherent nonlinear, strongly-coupled dynamics. Flight Envelope Protection (FEP) technology, which aims to mitigate these limitations by predicting potential limit exceedances and implementing effective avoidance measures, has emerged as a critical research area in modern helicopter flight control to reduce pilot workload, enhance flight safety, and maximize available maneuverability. This paper presents a comprehensive review of the key technologies and research progress in helicopter FEP. It begins by outlining the composition, classification, and typical limit parameters of the helicopter flight envelope. Subsequently, following the two fundamental stages of "envelope prediction" and "envelope avoidance," a variety of protection methods are elaborated. For envelope prediction, the principles and applicability of prominent algorithms such as the dynamic trim method, the peak response estimation method, and the data-driven method are analyzed. For envelope avoidance, cockpit-cueing-based protection strategies, automatic intervention architectures based on command limiting and control limiting, protected-variable-feedback methods, and approaches that integrate envelope constraints into controller design are reviewed. A comparative analysis based on simulation results highlights the distinct characteristics of these methods in terms of maneuverability, protection effectiveness, and robustness. Finally, the paper summarizes the primary challenges in the field, such as the special demands of advanced rotorcraft configurations; the trade-off between FEP and handling qualities and the robust protection under uncertainties and fault conditions. Future trends toward more intelligent, adaptive, and integrated FEP systems are also prospected. This review is intended to provide a valuable reference for both research and engineering applications in helicopter FEP technology.