The monograph is devoted to the systematic presentation of the theory of optical properties of crystals in the exciton region of the spectrum. Although it pays great attention to the theory of small radius excitons, the results of consideration of a number of issues raised in the book, such as as the theory of retardation effects, forms of exciton light absorption bands, phenomenological theory of energy transfer by excitons, phenomenological theory of surface excitons, theory of dielectric constant tensor, nonlinear polarizables in the exciton region of the spectrum and some others, are more general and can be used for excitons of arbitrary structure.