In recent years, considerable progress has been made in the study of organic semiconductors. The main directions of investigation, have been determined and substantial results have been achieved in the theoretical treatment of many questions. However, the range of investigations is so broad that it is impossible to discuss all branches fully in a short monograph. In fact, the chemist synthe­ sizing systems with conjugated bonds and studying their reactivity, the physical chemist studying the catalytic and absorption proper­ ties of substances with conjugated bonds, the physicist studying the photoelectric and magnetic characteristics of dyes and polynuclear hydrocarbons, and the biologist attempting to explain the mechan­ ism of energy transfer in the living cell -they are all working in the field of organic semiconductors. Consequentiy, in the present book the authors have not attempted to include all sides of the problem of organic semiconductors. The monograph considers in detail only those questions which appear to the authors to be most immediate and interesting. Other questions are touched on only to the extent necessary to illustrate the main material. The book begins with a statement of material which is of the nature of a review. The main classes of organic semiconductors are mentioned but the methods for their preparation are not illus­ trated in detail. Somewhat more attention is devoted to the depen­ dence of the electrical characteristics of organic semiconductors on their structure and chemical composition.