To achieve this, five different metal species are introduced, sharing the same nitrogen-coordinated site, thereby increasing the configurational entropy of the system beyond 1.5R. However, despite a huge number or research articles having been published in this area in the last ten years, much of the underlying science of these materials remains under debate. High entropy has become a commonplace term in materials research ever since a new method of alloy design was proposed in 2004, relying on the combination of multiple elements in equimolar ratios. Herein, a new approach is presented to substantially improve the electrochemical properties of PBAs by introducing high entropy into the crystal structure. Studies of such alloys have identified a number of promising properties, including high strength, good wear characteristics and excellent corrosion resistance, which make these materials industrially relevant. In recent years, the concept of entropy stabilization of crystal structures in oxide systems has led to an increased research activity in the field of 'high entropy oxides'. However, they exist in a relatively unex-plored region of quasi-limitless composition space. The authors summarize key properties of HEAs from the perspective of both fundamental understanding and applications, which are supported by in-depth analyses. To rationalise these observations, it was suggested that entropy of mixing in these systems must be very high, extending the mutual solubility of different elemental species and stabilising simple structured solid solutions with respect to the formation of intermetallic compounds. High-entropy alloys (HEAs) are a class of alloys that can exhibit promising properties including enhanced irradi-ation resistance, high-temperature strength, and corrosion resistance. This book provides a systematic and comprehensive description of high-entropy alloys (HEAs). Experimental studies have reported single or dual phase as-cast microstructures, and corresponding diffraction data have indicated that these phases have simple crystal structures, such as fcc or bcc. Traditional metallurgical wisdom would expect the microstructure of these materials to contain a number of intermetallic phases, yet surprisingly, this has not to been the case. Contrary to conventional alloying, these materials do not have a principal component, but are instead based on near equiatomic mixtures of five or more elements. High Entropy Alloys (HEAs) are an intriguing new class of metallic materials, based on a novel approach to materials design.