A description is given of the current state of the theory of intermolecular interactions. This includes a general physical picture of these interactions and an analysis of the relative contributions of different types of interaction (electrostatic, resonance, induction, dispersion, relativistic, magnetostatic, and exchange). The leading features of each contribution are discussed. A review is also given of methods available for calculating the potential specific for each range of distances between the interacting systems. Particular attention is devoted to an analysis of approximations used in different theoretical calculations.