The theoretical foundations of quantum chemical methods for calculating molecules, molecular systems and solids, as well as modern views on chemical bonding and intermolecular interactions are presented. Methods of interpreting the results of quantum chemical calculations and methods of calculating the properties of chemical substances are considered. The material required by both the research chemist and the process chemist for practical work in the conditions of modern high-tech industries is presented in an accessible form with a wide involvement of illustrations. For undergraduates, graduate students, doctoral students, teachers of chemistry departments of classical, pedagogical and technological universities, as well as for a wide range of specialists in various fields of chemistry, physics, biology and materials science.