The applicability of ab initio SCF and correlation energy calculations for the computation of intermolecular forces is discussed. The total interaction energy is divided into individual contributions by means of perturbation theory. The 1/R expansion is used to discuss the importance of the dispersion energy in comparison to those terms which are already included at the Hartree-Fock level. The importance of a correct description of molecular properties like multipole moments and polarizability is stressed in connection with a reliable calculation of interaction energies. Several selected examples illustrate these general considerations.