This book contains the transcripts of the lectures presented at the NATO Advanced Study Institute on "Electrons in Finite and Infinite Structures," held at the State University of Ghent, Belgium, August 30-September 11, 1976. Over the last few years substantial progress has been made in the description and the understanding of the behavior of electrons in extended bodies. This includes the study of the energy spectrum of electrons in large molecules, perfect as well as imperfect crys­ tals, and disordered alloys. Not only local potential techniques but also the many-body aspects are discussed in detail. As atomic, molecular, and solid state physics involve common techniques and insights, we believe that physicists and chemists active in these fields have benefited from these lectures and the interchange of ideas during the course. The aim of the Institute was to familiarize young scientists in the field with the current state of the art and to indicate in which areas advances may be expected in the near future. The A.S.I. consisted of two parts: detailed instructional and review lectures over the whole period and some evening sessions where the partici­ pants were offered the opportunity to present their own work and discuss their ideas with senior scientists. Since the Institute took place a few weeks after Prof. Dr. John C. Slater was suddenly taken from our scientific community, it was a great honor for us to dedicate this course, on behalf of the organizing committee, to the late John C. Slater.