The book contains an exposition of the basic ideas of the modern theory of disordered semiconductors. New ideas that have arisen in connection with the study of the motion of particles in random fields, the characteristics of which are given only statistically (the idea of ​ ​ "self-decreasing" values, the localization of electrons in random potential wells, and others) are discussed in detail. It turns out which representations of the standard zone theory remain valid when applied to disordered semiconductors. Methods for calculating the energy, electrical and optical characteristics of disordered systems developed in the last decade are considered (in particular, the method of optimal fluctuation, quasi-classical approximation and flow theory).