X-ray powder diffraction is one of the methods used for detection and analysis of polymorphic forms of pharmaceutical substances. The article elucidates the concept of polymorphism, briefly explains physical characteristics of this phenomenon, conditions of polymorphic transformations and the prevalence of polymorphic forms among drug substances. It should be noted that polymorphism is observed in drug substances belonging to different pharmacologic classes. Polymorphic forms of the same drug substance have different solubility, melting point, resistance to oxidation and to other destructive processes, and, consequently, different surface properties which affect both the rate of absorption of the drug substances and their stability as components of dosage forms. This calls for the need to control the quality of drug substances for potential presence of polymorphic forms. The use of diffraction methods for examination of cryomodified forms of various biologically active compounds obtained by evaporation and subsequent precipitation at low temperatures resulted in obtaining polycrystalline substances with new properties. The article provides results of examination of crystalline modifications of phenazepam in the form of α- and β-polymorphs, tilorone, fabomotizole, zolendronic acid and dehydroepiandrosterone. It was demonstrated that the use of X-ray diffraction analysis for examination and quality control of polymorphic forms of drugs is a necessary component of identification testing.