Since the introduction of quantum mechanics, the general theory of solid state physics has developed very rapidly. To date, a number of good textbooks on general solid state physics have been written. However, research in solid state physics has become highly specialized and undertaken in narrow fields. There is thus a great need for elementary textbooks that deal in detail with the study of solids in a particular field in order to give students basic knowledge in that field. Metallic solids with an impurity, generally called alloys, are of immense importance from both fundamental and technological points of view. The pioneering work of Bloember­ gen and Rowland (1953) gave considerable impetus to the study of the electronic structure of metallic alloys. Serious theoretical study in this field started in 1960 and, during the last two decades, considerable success in understanding the electronic structure of simple metal alloys has been achieved. Nonetheless the theoretical study of dilute alloys of transition metals is still in its infancy. At present there are few review articles and original research papers that examine the role of an impurity with respect to the electronic structure and properties of metallic alloys. Because of the absence of an elementary textbook that presents a comprehensive account of different aspects of the electronic structure of metallic alloys, I have written this elementary textbook on the theory of the electronic structure of metallic alloys.