A combination of different metals or metals which are combined with one or more elements is known as an alloy. Some common examples of such alloys are steel, sterling silver, nichrome, brass, bronze, etc. Alloys can be of two types either of a solid solution of metal elements consisting of a single phase or mixture of metallic phases including two or more solutions. The two main classifications of alloys are substitutional alloys and interstitial alloys. Substitutional alloys are those alloys in which some of the atoms composed of the metallic crystals are substituted with atoms of the other constituents. Interstitial alloys are those alloys in which one atom is smaller than the rest. It cannot be substituted for the other type of atom in the crystals. This book contains some path-breaking studies on alloys. It covers in detail some existent theories and innovative concepts revolving around alloys. This book is appropriate for students seeking detailed information in this area as well as for experts.