There are relatively few revolutions in the venerable and rather staid field of metallurgy. One can count among them the advent of metallic glasses, of superplastic metals, or of memory-alloys. The latest revolution involves the relatively staid topic of alloy formulation, but is all the more startling because the resultant materials break every long-cherished rule of alloy design. In particular, the famous empirical rules of Hume-Rothery are completely ignored. That is, in the archetypal high-entropy alloy, five metals are alloyed together in equal proportions regardless of atomic-size difference, valence or crystal structure. Commonsense would tell any experienced metallurgist that that could result only in a uselessly brittle mass of intermetallic compounds. But in a truly paradigm-shifting manner, Professor J.W.Yeh of Taiwan correctly predicted that a high configurational entropy could suppress the appearance of detrimental intermetallic compounds and lead to simple familiar microstructures having very useful properties. High-Entropy Alloys can exhibit, for instance, astounding hardness and strength and also have a very good corrosion resistance. The present book summarises the microstructures and properties of all of the high-entropy alloys.