Semiconductor technology has evolved rapidly since the invention of the transistor in the late 1940s by Bardeen, Brattain, and Shockley. With this progress has come a host of experiments and theories of a fundamental character concerning the electronic structure of covalent semiconductor materials. These developments have made it possible to understand the microscopic electronic structure of this family of materials more precisely and more systematically than that of any other assembly of interacting atoms. These advances can be described without the use of complicated mathematics or elaborate models. The description combines the language of solid state physics with that of chemistry and metallurgy. The utility of these approaches and their interrelations are apparent from the great extent to which the subject has developed.The level at which this book has been written is typical of that of many introductory books on solid state physics. The approach, however, is interdisciplinary because many results are described both in terms of the energy bands of the physicist and the covalent bonds of the chemist. There is less emphasis on mathematical derivations than on relations between structure and properties. In these respects the reader will find emphasized here many basic properties of materials that are often ignored or regarded as accidents of nature in the traditional approach.