The book contains a summary of the main ideas of the modern theory of disordered semiconductors. New ideas that have arisen in connection with the study of particle motion in random fields, the characteristics of which are given only statistically (the idea of quantities, the localization of electrons in random potential pits, etc.) are discussed in detail. It turns out which representations of the standard zone theory remain valid in the application to disordered semiconductors. Methods developed in the last decade for calculating the energy, electrical and optical characteristics of disordered systems (in particular, the method of optimal fluctuation, quasi-classical approximation and flow theory) are considered. The applications of these representations for solving a number of problems of kinetics and optics of disordered materials are also considered (electrical conduction and thermoeds in the field of jump conduction, inter-zone light absorption, combined scattering of light on electrons).