The knowledge of the characteristics and mechanisms of the interaction of the active centres of complex molecules is of primary importance for the development of theoretical and applied organic chemistry, promotes a more productive utilisation of the enormous amount of experimental data available, facilitates predictions and the search for new routes to and new varieties of the synthesis, industrial technology, and applications of organic compounds, and leads to more complete ideas about their functions in chemical, physical, and physiological processes. The studies in this field have been subjected to a critical survey and attention has been paid to the possibility of distinguishing different forms of the mutual influence of atomic groups, the establishment of correlations between different parameters of conjugation and of the role of relativistic effects, the characteristics of conjugation systems incorporating heteroatoms from different groups in the Periodic System, and the study of the evolution of excited molecules. The bibliography includes 188 references.