High entropy alloys (HEAs) and the more broadly defined multi-principal-elements alloys (MPEAs) represent a major paradigm shift in alloy design. HEAs focus on the compositions near the center of a multicomponent phase diagram. By contrast, traditional alloys focus compositions on the boundaries (vertices, edges, or faces) of a phase diagram and typically consist of one principal element. Therefore, HEAs represent a tremendously vast compositional space that is largely unexplored by science. As an emerging field, research on HEAs has attracted rising worldwide attention and interest from both academia and industry since 2004. The number of published papers has increased rapidly each year, and there have been many dedicated conferences, symposia and workshops on HEAs. Traditional physical metallurgy principles as well as novel processing methods have all been applied to HEAs, and new materials with extraordinary properties have been reported. New results also show that traditional materials science concepts are inadequate to explain some of the newly observed behaviors, fueling intense development of new models for complex, concentrated alloys. The high-entropy concept has now been extended to ceramics, semiconductors, polymers, and a broad range of functional materials. As a result, the whole field has advanced dynamically and rapidly in almost every aspect of materials science and engineering.