[en] The review of the publications devoted to the study of segregation is given. The causes of segregation are discussed. It is shown that the tendency of the activator towards increase of the covalency of bonding with its surroundings results under chemical differentiation conditions in deviation from random distribution of the activator in the glass. The spectroscopic symptoms of segregation are discussed on the examples of the glasses subjected to RZI activation, photochromous magnetoactive glasses, etc. The methods of controlling segregation processes and possible fields of segregation application are considered