Abstract There have been many developments in the theory of intermolecular forces over the last twenty years or so that have not so far been collected together in book form. After many years of effort, practical methods have emerged for the accurate calculation of intermolecular interactions by perturbation theory. Increasing computer power is making it possible to pension off the Lennard-Jones potential after 75 years of valuable service and to use more realistic models; similarly point-charge models are giving way to more so phisticated multipole descriptions. This has come at a time when experimental methods for determining intermolecular potentials are becoming more and more powerful and re liable, and when interest in accurate models of intermolecular interactions is greater than ever before, not least because of the increasing use of computer modelling for simulating systems of biological interest and for designing biologically active molecules for use as drugs and pesticides and the like.