High-entropy alloys (HEAs) are multicomponent solid solution systems that combine five or more elements in near-equiatomic proportions. These alloys have four basic effects that influence their behavior: high-entropy, cocktail (diversity), lattice (structure) distortion, and sluggish (slow) diffusion effects. High-entropy lowers Gibbs free energy, promoting single-phase solid solutions and microstructural stability. The cocktail effect improves mechanical, physical, and chemical properties of HEAs. Variations in the size of atoms create stress fields inside the crystal lattice, preventing dislocation movement and enhancing mechanical strength. Sluggish diffusion behavior in HEAs enhances microstructural stability at elevated temperatures. HEAs have superior properties such as high oxidation and corrosion resistance, excellent wear resistance and good strength. In this chapter, HEAs, their properties, the main parameters determining their properties, the processes involved and their impact on material performance, and their potential uses in advanced engineering applications are reviewed and presented.