The possibilities of modelling of irregular interstratified structures of single crystals with chemical inhomogeneity are discussed: the theoretical basis, the requirements to the X-ray experiment, the accuracy and reproducibility of the diffraction data, the reliability of the suggested models. The examples of the successful application of the approach to the study and description of the defect structures of single crystals belonging to different structural types are considered. Three general types of irregular interstratified structures differing in the nature of chemical inhomogeneity as well as in the peculiarities of their diffraction patterns are distinguished.