High-entropy alloys (HEAs) are a new generation of metallic alloys that are composed of multiple principal elements, typically five or more, in near-equiatomic proportions. However, recently, alloys with fewer principal elements but similar compositional complexity have also been classified as HEAs. Interestingly, the non-conventional composition of HEAs differs from that of conventional alloy structures, where one or two major elements form the base. This unusual compositional complexity of HEAs leads to extraordinary properties, such as ultra-high strength, excellent stability against temperature variations, better wear resistance, and good corrosion behavior. These characteristics result from four fundamental effects that affect phase formation and microstructural stability collectively. These four core effects (high-entropy effect, sluggish diffusion effect, severe lattice distortion effect, and cocktail effect) have been presented in the current chapter. With potential uses in energy, aerospace, automobile, and structural components, HEAs have emerged as a revolutionary field in materials science and engineering due to the increased interest in them. The basic ideas, phase evolution, and mechanical characteristics of HEAs at various temperatures are thoroughly discussed in this chapter.