High-entropy alloys (HEAs) are alloys with five or more principal elements, characterized by high mixing entropy that promotes the formation of simple solid solution phases. This unique composition leads to exceptional properties such as high strength, wear resistance, high-temperature strength, and corrosion resistance. HEAs have gained significant attention due to their potential applications in various fields. This review discusses key aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, and corrosion behaviors. It also highlights current challenges and future directions in the field. Core effects in HEAs include the high-entropy effect, which lowers the free energy of solid solution phases and facilitates their formation. The sluggish diffusion effect slows down diffusion and phase transformation kinetics in HEAs. The severe-lattice-distortion effect leads to lattice distortion, which can hinder dislocation movement and increase hardness. The cocktail effect refers to the interaction between elements, which can influence the alloy's properties. HEAs can form various phases, including simple solid solutions and intermetallic compounds. The phase stability in HEAs is influenced by factors such as enthalpy and entropy of mixing, atomic size differences, and valence electron concentration (VEC). The VEC determines the phase stability, with higher VEC favoring FCC structures and lower VEC favoring BCC structures. High-entropy bulk metallic glasses (HEBMGs) are a special type of HEA that can form amorphous structures. They have unique properties such as high crystallization temperatures and good diffusion barrier performance. The phase evolution in HEAs at elevated temperatures is influenced by factors such as annealing temperature and composition. The mechanical properties of HEAs are closely related to their phases. HEAs can exhibit high hardness, strength, and ductility. The Al-Co-Cr-Cu-Fe-Ni alloy system is a well-studied example, with phases such as FCC, BCC, and Cu-rich phases. The addition of elements like Ti, Mo, and Mn can lead to the formation of intermetallic phases, which can enhance the alloy's properties. HEAs also exhibit excellent wear and fatigue properties. The Al-Co-Cr-Cu-Fe-Ni alloy has been shown to have good wear resistance and fatigue life. The high-temperature properties of HEAs are also promising, with some HEAs maintaining high strength at elevated temperatures. Overall, HEAs have shown great potential in various applications due to their unique properties and the ability to be fabricated with existing technologies. However, challenges remain in understanding and controlling their phase evolution, mechanical properties, and high-temperature performance. Further research is needed to fully explore the potential of HEAs.High-entropy alloys (HEAs) are alloys with five or more principal elements, characterized by high mixing entropy that promotes the formation of simple solid solution phases. This unique composition leads to exceptional properties such as high strength, wear resistance, high-temperature strength, and corrosion resistance. HEAs have gained significant attention due to their potential applications in various fields. This review discusses key aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, and corrosion behaviors. It also highlights current challenges and future directions in the field. Core effects in HEAs include the high-entropy effect, which lowers the free energy of solid solution phases and facilitates their formation. The sluggish diffusion effect slows down diffusion and phase transformation kinetics in HEAs. The severe-lattice-distortion effect leads to lattice distortion, which can hinder dislocation movement and increase hardness. The cocktail effect refers to the interaction between elements, which can influence the alloy's properties. HEAs can form various phases, including simple solid solutions and intermetallic compounds. The phase stability in HEAs is influenced by factors such as enthalpy and entropy of mixing, atomic size differences, and valence electron concentration (VEC). The VEC determines the phase stability, with higher VEC favoring FCC structures and lower VEC favoring BCC structures. High-entropy bulk metallic glasses (HEBMGs) are a special type of HEA that can form amorphous structures. They have unique properties such as high crystallization temperatures and good diffusion barrier performance. The phase evolution in HEAs at elevated temperatures is influenced by factors such as annealing temperature and composition. The mechanical properties of HEAs are closely related to their phases. HEAs can exhibit high hardness, strength, and ductility. The Al-Co-Cr-Cu-Fe-Ni alloy system is a well-studied example, with phases such as FCC, BCC, and Cu-rich phases. The addition of elements like Ti, Mo, and Mn can lead to the formation of intermetallic phases, which can enhance the alloy's properties. HEAs also exhibit excellent wear and fatigue properties. The Al-Co-Cr-Cu-Fe-Ni alloy has been shown to have good wear resistance and fatigue life. The high-temperature properties of HEAs are also promising, with some HEAs maintaining high strength at elevated temperatures. Overall, HEAs have shown great potential in various applications due to their unique properties and the ability to be fabricated with existing technologies. However, challenges remain in understanding and controlling their phase evolution, mechanical properties, and high-temperature performance. Further research is needed to fully explore the potential of HEAs.