The electronic properties of strongly doped semiconductors (SDP) differ greatly from the properties of pure semiconductors, to which many monographs and text books have been devoted. A serious study of SDP was started about 10 years ago. The quasiclassical method, its results, and the region of applicability are discussed in detail. It is shown that this method cannot be used to describe deeplylying fluctuation levels. This is followed by the recently developed method of optimal fluctuation, which makes it possible to find the argument of the exponential of the density of states deep in the forbidden band. Using this method, the density of states is analyzed at different relations between the semiconductor parameters and at all values of the energy for which the effective-mass method is applicable. The difference between the spectra of the majority and minority carriers is indicated. The influence exerted on the state density by the correlation in the impurity distribution is considered. A study of the structure of the fluctuation levels makes it possible to construct the theory of the interband light absorption coefficient (ILAC) at frequencies below the threshold. It is shown that the frequency dependence of the ILAC does not always duplicate the dependence of the density of states on the energy. An analysis of the experimental data indicates that the minority carriers play an important role in the formation of the ``tail of the ILAC''.