High-entropy alloys (HEAs) are alloys containing five or more principal elements, each with a concentration between 5 and 35 at.% and minor elements below 5 at.%. These alloys exhibit unique properties due to their high mixing entropy, which facilitates the formation of simple solid solution phases and reduces the number of phases compared to conventional alloys. The paper reviews critical aspects of HEAs, including core effects, phase and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Core effects include the high-entropy effect, which lowers the free energy of solid solution phases, the sluggish diffusion effect, which slows down diffusion and phase transformation kinetics, the severe-lattice-distortion effect, which impairs dislocation movement, and the cocktail effect, which considers the interactions among component elements. Phase and crystal structures are classified into simple disordered phases (SDPs), simple ordered phases (SOPs), and complex ordered phases (COPs). Mechanical properties are influenced by the hardness/strength of each phase, their relative volume ratios, and their morphology/distribution. High-temperature properties and structural stability are enhanced due to the sluggish diffusion effect, reduced driving force for defect elimination, and good resistance to thermal softening. Refractory HEAs, composed of refractory elements, show exceptional high-temperature strength and plasticity. Wear and fatigue properties are also promising, with some HEAs outperforming conventional materials. Overall, HEAs offer exciting potential for various applications due to their unique properties and ease of fabrication.High-entropy alloys (HEAs) are alloys containing five or more principal elements, each with a concentration between 5 and 35 at.% and minor elements below 5 at.%. These alloys exhibit unique properties due to their high mixing entropy, which facilitates the formation of simple solid solution phases and reduces the number of phases compared to conventional alloys. The paper reviews critical aspects of HEAs, including core effects, phase and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Core effects include the high-entropy effect, which lowers the free energy of solid solution phases, the sluggish diffusion effect, which slows down diffusion and phase transformation kinetics, the severe-lattice-distortion effect, which impairs dislocation movement, and the cocktail effect, which considers the interactions among component elements. Phase and crystal structures are classified into simple disordered phases (SDPs), simple ordered phases (SOPs), and complex ordered phases (COPs). Mechanical properties are influenced by the hardness/strength of each phase, their relative volume ratios, and their morphology/distribution. High-temperature properties and structural stability are enhanced due to the sluggish diffusion effect, reduced driving force for defect elimination, and good resistance to thermal softening. Refractory HEAs, composed of refractory elements, show exceptional high-temperature strength and plasticity. Wear and fatigue properties are also promising, with some HEAs outperforming conventional materials. Overall, HEAs offer exciting potential for various applications due to their unique properties and ease of fabrication.